High-strength high-plasticity steel for ultra-low temperature environment and preparation method thereof

Abstract

The application belongs to the technical field of ultralow-temperature materials, and discloses a high-strength and high-plasticity steel for an ultralow-temperature environment and a preparation method thereof. A high-manganese and high-nitrogen component system is adopted, based on the austenite stabilization theory, the expensive nickel element is replaced by the low-cost manganese element to realize austenite stabilization, and through the promoting effect of manganese and chromium elements on nitrogen solubility, a synergistic mechanism of high-manganese stable phase and high-nitrogen strengthening is formed. The heating and finish-forging temperature is strictly controlled in the forging stage to ensure the uniformity of the structure, the homogenization treatment is performed before hot rolling to promote the full solid solution of manganese and nitrogen, and the finish-rolling temperature is accurately controlled by adopting the controlled rolling process to retain high-proportion deformed austenite grains to further improve the material strength. While reducing the cost of more than 75% per ton of alloy, the performance of the ultralow-temperature material is greatly improved, and excellent strength and plasticity matching of the yield strength ≥ 1600 MPa, the tensile strength ≥ 1900 MPa and the elongation ≥ 20% at-269 ℃ is obtained.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of ultralow-temperature materials, and particularly relates to a high-strength high-plasticity steel for ultralow-temperature environment and a preparation method thereof. BACKGROUND

[0002] With the rapid development of cutting-edge technologies such as deep space exploration and fusion reactor superconducting magnets, and the sharp rise in demand for ultralow-temperature energy such as liquefied natural gas, liquid hydrogen and liquid helium in the storage and transportation fields, unprecedented stringent requirements have been placed on the performance of materials serving in ultralow-temperature environments. Such materials not only need to have high strength, excellent low-temperature toughness and plasticity at extremely low temperatures, but also need to have good corrosion resistance, and the thermal expansion coefficient needs to be strictly controlled to ensure structural stability. In addition, some scenarios also have special requirements for the non-magnetic properties of the materials.

[0003] In the field of ultralow-temperature materials, traditional solutions mostly rely on high-nickel austenitic stainless steel, the core principle of which is to stabilize the austenitic phase through high content of nickel element to improve the low-temperature performance. For example, the Chinese patent document CN117286426A discloses "High-strength non-magnetic austenitic stainless steel bar suitable for nuclear fusion armor", which can achieve a yield strength of 1250MPa, a tensile strength of 1700MPa and an elongation of 25% at -269℃, but its nickel content is as high as 14.0%~15.5%. However, nickel is a scarce and expensive metal, and its high cost seriously restricts the application of this type of material in large-scale engineering, and the process mostly uses a single processing method, which is difficult to flexibly adjust the strength and plasticity of the material according to different working conditions, and has limitations in performance adjustment. 1250MPa, a tensile strength 1700MPa, and an elongation 25% at -269℃, but its nickel content is as high as 14.0%~15.5%. However, nickel is a scarce and expensive metal, and its high cost seriously restricts the application of this type of material in large-scale engineering, and the process mostly uses a single processing method, which is difficult to flexibly adjust the strength and plasticity of the material according to different working conditions, and has limitations in performance adjustment.

[0004] In view of the outstanding problems of existing ultralow-temperature materials such as performance deficiency, strength and plasticity inversion, and high cost, the present application provides a high-strength high-plasticity steel for ultralow-temperature environment and a preparation method thereof, which has low alloy cost and good strength and plasticity and non-magnetic properties at ultralow temperature. SUMMARY

[0005] In view of the prior art problems, in the aspect of component design, the application adopts a high-manganese and high-nitrogen component system, realizes austenite stabilization by replacing expensive nickel element with low-cost manganese element based on the austenite stabilization theory, and forms a synergistic mechanism of high-manganese stable phase and high-nitrogen strengthening through the promoting effect of manganese and chromium elements on nitrogen solubility. In the aspect of process, the application strictly controls heating and finish forging temperature to ensure uniformity in the forging stage, promotes full solid solution of manganese and nitrogen through homogenization treatment before hot rolling, and accurately controls the finish rolling temperature at 820-880 DEG C through controlled rolling process to retain high proportion of deformed austenite grains to further improve the strength of the material, and then combines the optional schemes of "water cooling direct forming" and "air cooling + offline solid solution", so that the yield strength of the alloy at-269 DEG C 1600MPa, tensile strength 1900MPa and elongation 20% excellent strength and ductility matching, provides a low-cost, high-performance, easy-to-produce and flexible control material solution for super-low-temperature engineering equipment, and has great engineering application value and broad market prospect.

[0006] The technical scheme of the application is as follows: a high-strength and high-ductility steel for super-low-temperature environment, the chemical composition of the high-strength and high-ductility steel for super-low-temperature environment contains, by mass percentage, C 0.03%; Mn: 20.0%-28.0%; Cr: 10.0%-15.0%; Si: 0.20%-0.40%; N: 0.25%-0.40%; Cu 1.0%; Nb 0.20%; V 0.20%; S 0.005%; P 0.005%; the balance is Fe and inevitable impurities.

[0007] The mass percentages of Mn, Cr, N and Si elements satisfy the following relationship:

[0008] ; .

[0009] The high-strength and high-ductility steel for super-low-temperature environment has a relative magnetic permeability of <1.03 under-269 DEG C and 5T magnetic field intensity.

[0010] The high-strength and high-ductility steel for super-low-temperature environment has a stable single-phase austenite structure, and no martensite phase is generated under-269 DEG C through X-ray diffraction or EBSD analysis detection.

[0011] The grain with a local orientation difference greater than 0.65° is defined as a deformed austenite grain, and the volume fraction of the deformed austenite grain in the base structure of the high-strength and high-plasticity steel for super-low-temperature environment is greater than or equal to 30% and less than or equal to 90%.

[0012] The high-strength and high-plasticity steel for super-low-temperature environment has a yield strength greater than 550 MPa, a tensile strength greater than 800 MPa, and an elongation greater than 35% when stretched at room temperature.

[0013] The high-strength and high-plasticity steel for super-low-temperature environment has a yield strength greater than 1250 MPa, a tensile strength greater than 1600 MPa, and an elongation greater than 35% when stretched at -196 ℃.

[0014] The high-strength and high-plasticity steel for super-low-temperature environment has a yield strength greater than 1600 MPa, a tensile strength greater than 1900 MPa, and an elongation greater than 20% when stretched at -269 ℃.

[0015] A preparation method of a high-strength and high-plasticity steel for super-low-temperature environment, comprising the following steps:

[0016] Step 1, smelting: smelting according to the composition of the high-strength and high-plasticity steel for super-low-temperature environment to obtain an alloy ingot, and ensuring that the nitrogen content in the alloy ingot is 0.25wt%-0.40wt% after smelting is completed;

[0017] Step 2, forging: performing forging treatment on the alloy ingot to obtain a forged blank; the heating temperature of the forging is 1070 ℃-1150 ℃, and the final forging temperature is 950 ℃-1030 ℃;

[0018] Step 3, heating: performing homogenization treatment on the forged blank before hot rolling to ensure that manganese and nitrogen elements are fully solid-solved, and the heating temperature is 1200 ℃-1250 ℃ and the time is 2h-4h;

[0019] Step 4, hot rolling: performing hot rolling on the homogenization-treated forged blank to obtain a plate; the opening rolling temperature is 1050 ℃-1150 ℃, the final rolling temperature is 820 ℃-880 ℃, and the single-pass reduction rate of the hot rolling is not greater than 30%;

[0020] Step 5, cooling: water cooling or air cooling the hot-rolled plate to room temperature to obtain the high-strength and high-plasticity steel for super-low-temperature environment.

[0021] After the air cooling to room temperature, offline solid solution treatment at 880 ℃-950 ℃ is added, and the time is 5min-15min.

[0022] The smelting mode is vacuum consumable or electroslag remelting:

[0023] The vacuum consumable is used to reduce the escape of nitrogen elements due to the increase of vacuum degree by controlling the argon partial pressure, so as to ensure the nitrogen yield 80%;

[0024] The electroslag remelting adopts a quaternary slag system, and is carried out under a protective atmosphere of dry air.

[0025] Compared with the prior art, the high-strength high-plasticity steel for ultra-low temperature environment and the preparation method thereof have the following advantages and beneficial effects:

[0026] 1. The high-strength high-plasticity steel for ultra-low temperature environment provided by the present application adds 10.0% to 15.0% of chromium element by mass percentage, the chromium element can cooperate with the manganese element to improve the solid solubility of the nitrogen element in the alloy, and the yield strength and the tensile strength of the alloy are significantly improved through the solid solution strengthening of 0.25% to 0.40% of high nitrogen element, in addition, the addition of the chromium element can also improve the corrosion resistance of the high-strength high-plasticity steel in the ultra-low temperature, and 0.03% of ultra-low carbon design of carbon atom mass percentage is adopted to avoid the generation of brittle carbides or inclusions such as M 23 C6 in the hot forming process, and 1% of copper element by mass percentage is added to improve the hydrogen embrittlement resistance of the high-strength high-plasticity steel in the ultra-low temperature, and the yield strength of the high-strength high-plasticity steel is greater than 1600 MPa, the tensile strength is greater than 1900 MPa, the elongation is greater than 20%, and the magnetic permeability is less than 1.03, so that the high-strength high-plasticity steel has excellent characteristics of high strength, high plasticity and non-magnetic in the ultra-low temperature. 2. The preparation method of the high-strength high-plasticity steel for ultra-low temperature environment provided by the present application has simple process, only five steps of smelting, forging, heating, hot rolling and cooling, and is easy to industrialize. In addition, through the optional process paths of “water cooling direct forming” and “air cooling + offline solid solution”, the proportion of deformed austenite grains and equiaxed austenite grains in the material can be flexibly controlled: the water cooling scheme retains more deformed austenite, and higher strength is realized through dislocation strengthening; after air cooling, offline solid solution at 880 DEG C to 950 DEG C for a short time can increase the proportion of equiaxed austenite grains, significantly improve the elongation, realize the accurate control of strength and plasticity, and meet the individual needs of different ultra-low temperature working conditions.

[0027] 3. The high-strength high-plasticity steel for ultra-low temperature environment provided by the present application adopts a high-manganese nickel-free component system design, and uses the low-cost manganese element to replace the expensive nickel element to stabilize the austenite, compared with the traditional high-nickel type austenitic stainless steel for ultra-low temperature, the cost of the component system alloy is reduced by more than 75%, which significantly reduces the use cost of the ultra-low temperature material.

[0028] 3. The high-strength high-plasticity steel for ultra-low temperature environment provided by the present application adopts a high-manganese nickel-free component system design, and uses the low-cost manganese element to replace the expensive nickel element to stabilize the austenite, compared with the traditional high-nickel type austenitic stainless steel for ultra-low temperature, the cost of the component system alloy is reduced by more than 75%, which significantly reduces the use cost of the ultra-low temperature material.

[0029] ​In summary, the present application realizes further breakthrough of super-low temperature performance while greatly reducing alloy cost by synergistic innovation of high manganese nickel-free component design, high nitrogen solid solution strengthening and controllable rolling process, and provides a low-cost, high-performance, easy-to-produce and flexible control material solution for super-low temperature engineering equipment, which has great engineering application value and broad market prospect. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 The EBSD diagram of the microstructure of the high-strength high-plasticity steel for super-low temperature environment prepared in Example 1 of the present application is shown, wherein (a) is a pole figure, and (b) is a local orientation difference distribution diagram.

[0031] Figure 2 The EBSD diagram of the microstructure of the high-strength high-plasticity steel for super-low temperature environment prepared in Example 2 of the present application is shown, wherein (a) is a pole figure, and (b) is a local orientation difference distribution diagram.

[0032] Figure 3 The EBSD diagram of the microstructure of the high-strength high-plasticity steel for super-low temperature environment prepared in Example 3 of the present application is shown, wherein (a) is a pole figure, and (b) is a local orientation difference distribution diagram.

[0033] Figure 4 The EBSD diagram of the microstructure of the high-strength high-plasticity steel for super-low temperature environment prepared in Example 4 of the present application is shown, wherein (a) is a pole figure, and (b) is a local orientation difference distribution diagram.

[0034] Figure 5 The EBSD diagram of the microstructure of the high-strength high-plasticity steel for super-low temperature environment prepared in Example 5 of the present application is shown, wherein (a) is a pole figure, and (b) is a local orientation difference distribution diagram.

[0035] Figure 6 The EBSD diagram of the microstructure of the material prepared in Comparative Example 1 of the present application is shown, wherein (a) is a pole figure, and (b) is a local orientation difference distribution diagram.

[0036] Figure 7 The EBSD diagram of the microstructure of the material prepared in Comparative Example 2 of the present application is shown, wherein (a) is a pole figure, and (b) is a local orientation difference distribution diagram.

[0037] Figure 8 The EBSD diagram of the microstructure of the material prepared in Comparative Example 3 of the present application is shown, wherein (a) is a pole figure, and (b) is a local orientation difference distribution diagram.

[0038] Figure 9 The EBSD diagram of the microstructure of the material prepared in Comparative Example 4 of the present application is shown, wherein (a) is a pole figure, and (b) is a local orientation difference distribution diagram. DETAILED DESCRIPTION

[0039] The application provides a high-strength high-plasticity steel for an ultralow-temperature environment, which has stable single-phase austenite organization and does not have martensite transformation at an ultralow temperature (-196 ℃ and -269 ℃).

[0040] Specifically, the high-strength high-plasticity steel provided by the application has the following chemical components in percentage by mass: C 0.03%; Mn: 20.0%-28.0%; Cr: 10.0%-15.0%; Si: 0.20%-0.40%; N: 0.25%-0.40%; Cu 1.0%; Nb 0.20%; V 0.20%; S 0.005%; P 0.005%; and the balance is Fe and inevitable impurities.

[0041] The high-strength high-plasticity steel for an ultralow-temperature environment provided by the application is designed with an ultralow-carbon composition, is processed by vacuum consumable or electroslag remelting, and uses high-purity metal raw materials, so that the carbon content is strictly controlled 0.03%, so as to avoid the formation of brittle carbides during hot forming, prevent the weakening of the grain boundary and the decrease of low-temperature toughness.

[0042] The high-strength high-plasticity steel for an ultralow-temperature environment provided by the application contains 20.0%-28.0% of manganese in percentage by mass. Manganese is a main expanding element in the austenite phase region, cooperates with nitrogen to ensure the formation of stable single-phase austenite organization at -269 ℃. As a low-cost alternative element of nickel, the price of manganese is only 1 / 10 of that of nickel, which greatly reduces the alloy cost of each ton of material. Meanwhile, manganese can play a certain solid solution strengthening effect to improve the low-temperature strength, and increase the solid solubility of nitrogen in austenite, forming a synergistic mechanism of high-manganese stable phase and high-nitrogen strengthening.

[0043] The high-strength high-plasticity steel for an ultralow-temperature environment provided by the application contains 10.0%-15.0% of chromium in percentage by mass. Chromium can significantly improve the corrosion resistance of the material in a liquid hydrogen or liquid helium environment by forming a dense passivation film, so as to meet the corrosion resistance requirement of an ultralow-temperature container. In addition, chromium cooperates with manganese to further improve the solid solubility of nitrogen in austenite and enhance the strengthening effect of nitrogen. Meanwhile, chromium can inhibit austenite recrystallization, cooperate with a controlled rolling process to refine the grains, form a fine deformed structure, and synergistically improve the strength and plasticity.

[0044] ​The high-strength high-plasticity steel for ultra-low temperature environment provided by the application comprises 0.25-0.40% of nitrogen by mass percentage. Nitrogen exists in the form of interstitial solid solution in the austenite lattice, and produces a strong solid solution strengthening effect. Meanwhile, nitrogen, as a strong austenite forming element, cooperates with manganese to stabilize a single-phase structure, suppresses brittle phase transition at low temperature, and does not introduce a magnetic phase, thereby ensuring the non-magnetic property of the material at ultra-low temperature. The solid solution strengthening effect is particularly significant at liquid helium temperature, and is a key to break through the strength bottleneck of traditional materials.

[0045] The high-strength high-plasticity steel for ultra-low temperature environment provided by the application comprises 0.20-0.40% of silicon by mass percentage. Silicon, as a deoxidizer in the smelting process, plays a role in reducing the oxygen content of molten steel and improving purity. Meanwhile, a small amount of silicon solid-solutes in the austenite, and can provide limited solid solution strengthening effect, but the content needs to be strictly controlled 0.40% to avoid the formation of ferrite and damage to low-temperature plasticity.

[0046] The high-strength high-plasticity steel for ultra-low temperature environment provided by the application comprises 0.20-0.40% of silicon by mass percentage. 1.0% of copper. Copper inhibits hydrogen atom diffusion by segregating at grain boundaries, and significantly improves the hydrogen embrittlement resistance of the material in a liquid hydrogen environment. In addition, a small amount of copper can be strengthened by solid solution or precipitation phase assistance, but the content needs to be controlled to balance plasticity, and is suitable for hydrogen-containing extreme working conditions.

[0047] The high-strength high-plasticity steel for ultra-low temperature environment provided by the application comprises 0.20-0.40% of silicon by mass percentage. 0.005% of phosphorus.

[0048] The high-strength high-plasticity steel for ultra-low temperature environment provided by the application comprises 0.20-0.40% of silicon by mass percentage. 0.005% of sulfur. Sulfur and phosphorus, as harmful impurities, easily segregate at grain boundaries to induce embrittlement, and need to be smelted with pure metal materials and strictly controlled in content to avoid a decrease in plasticity at ultra-low temperature, so as to meet the stringent requirements of extreme environments on material reliability.

[0049] Further, the content of Mn, Cr, N and Si elements needs to satisfy the following relationship to ensure the stability of austenite and the performance of the material:

[0050] (1)

[0051] (2)

[0052] Wherein, the content of each element is in mass percentage.

[0053] The limitation of formula (1) can balance the effect relationship between Mn, N austenite forming elements and Cr, Si ferrite forming elements, which can effectively ensure the dominance of austenite phase in the alloy, inhibit the precipitation of ferrite and other harmful phases, and thus improve the plasticity, toughness and low temperature stability of the alloy.

[0054] The limitation of formula (2) can ensure that the total amount of austenite forming elements and functional elements such as chromium and silicon in the alloy reaches the threshold of synergistic effect. This limitation can ensure that the alloy has stable austenite structure and at the same time meets the required mechanical properties, avoiding performance degradation due to insufficient total amount of elements.

[0055] Further, the high-strength high-plasticity steel for ultra-low temperature environment defined as the deformed austenite grain with a local orientation difference greater than 0.65°, and the volume fraction of the deformed austenite grain in the matrix structure is 30% and ≤90%.

[0056] Further, the relative magnetic permeability of the high-strength high-plasticity steel for ultra-low temperature environment is <1.03.

[0057] Further, the high-strength high-plasticity steel for ultra-low temperature environment has a yield strength of >550MPa, a tensile strength of >800MPa, and an elongation of >35% at room temperature; a yield strength of >1250MPa, a tensile strength of >1600MPa, and an elongation of >35% at -196℃; and a yield strength of >1600MPa, a tensile strength of >1900MPa, and an elongation of >20% at -269℃.

[0058] The preparation method of the high-strength high-plasticity steel for ultra-low temperature environment includes the following steps:

[0059] Step 1, smelting:

[0060] The alloy ingot is obtained by smelting according to the composition of the high-strength high-plasticity steel for ultra-low temperature environment, and the nitrogen content in the alloy ingot after smelting is 0.25wt%-0.40wt%;

[0061] Step 2, forging:

[0062] The alloy ingot is subjected to forging treatment to obtain a forged blank; the heating temperature of forging is 1070℃-1150℃, and the final forging temperature is 950℃-1030℃.

[0063] Step 3, heating:

[0064] The forged blank is subjected to homogenization treatment before hot rolling to ensure that elements such as manganese and nitrogen are fully solid-solved, and the heating temperature is 1200℃-1250℃ and the time is 2-4h;

[0065] Step 4, hot rolling:

[0066] The homogenized forging blank is hot-rolled, the starting rolling temperature is 1050-1150 DEG C, the final rolling temperature is 820-880 DEG C, and the single-pass reduction rate of the hot-rolling is not more than 30%.

[0067] Step 5, cooling:

[0068] The hot-rolled plate is water-cooled or air-cooled to room temperature to obtain the high-strength high-plasticity steel for ultra-low temperature environment.

[0069] In step 5, after the air-cooling to room temperature, offline solid solution treatment at 880 DEG C-950 DEG C for 5-15 min can be added.

[0070] In the application, the smelting method is vacuum consumable or electroslag remelting.

[0071] The vacuum consumable is controlled by argon partial pressure to reduce the escape of nitrogen due to the increase of vacuum degree, and ensure the nitrogen yield 80%.

[0072] The electroslag remelting adopts a quaternary slag system, is carried out under a protective atmosphere, and the protective atmosphere is dry air.

[0073] In order to make the purpose, technical scheme and advantages of the embodiments of the present application more clear, the present application is further described below in combination with embodiments and drawings. Obviously, the described embodiments are only part of the embodiments of the present application, not all the embodiments. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0074] Various embodiments of the present application can exist in the form of a range; it should be understood that the description in the form of a range is only for the convenience and brevity, and should not be understood as a hard limit on the scope of the present application; therefore, it should be considered that the described range has been specifically disclosed all possible sub-ranges and single values in the range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers in the range, such as 1, 2, 3, 4, 5 and 6, regardless of the range. In addition, whenever a numerical range is indicated in this document, it refers to any cited number (fraction or integer) in the indicated range. A and B exist alone, and B exists alone. Wherein A, B can be singular or plural.

[0075] In this invention, the terms "comprising" and "including" mean "including but not limited to". In this document, "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can mean that A exists alone. Furthermore, in the following embodiments, each embodiment focuses on the differences from other embodiments; similar or identical parts between embodiments can be referred to interchangeably.

[0076] In the following examples, hot rolling was carried out at the National Key Laboratory of Digital Steel, Northeastern University. The experiment was conducted on a two-roll reversible hot rolling mill. It should be noted that all process equipment or apparatus not specifically specified are conventional equipment or apparatus within the art. The implementation methods in the following embodiments, where specific conditions are not specified, were carried out according to conventional methods and conditions, and this invention does not impose specific limitations on them.

[0077] Example 1

[0078] In this embodiment, the method for preparing high-strength, high-ductility steel for ultra-low temperature environments includes the following steps:

[0079] Step 1, Smelting:

[0080] The smelting process in this embodiment adopts electroslag remelting. The quaternary slag system is as follows: Electrodes are made from pre-smelted metal raw materials and vertically inserted into a copper water-cooled crystallizer containing molten slag. Under a protective atmosphere, current flows through the electrodes, slag pool, and molten metal pool to form a circuit. The electrode tip is gradually melted by the resistance heat of the slag pool. As the molten metal passes through the slag pool in droplet form, it comes into full contact with the slag. Non-metallic inclusions are adsorbed by the slag, while harmful elements such as sulfur and phosphorus are removed through the slag-steel reaction. The molten metal is forced to cool within the crystallizer, resulting in bottom-up directional solidification, ultimately yielding a smooth-surfaced, dense-internal electroslag remelted ingot.

[0081] The measured composition of the electroslag remelted ingot after smelting is as follows (wt%, S=0.003, P=0.004, Fe balance).

[0082]

[0083] Step 2, Forging:

[0084] The electroslag remelted ingot was forged at a starting forging temperature of 1120℃ and a final forging temperature of 980℃ to obtain a rectangular forging ingot.

[0085] Step 3, heating:

[0086] The forging billet is placed in a box-type resistance furnace for homogenization treatment before hot rolling to eliminate component segregation of the forging billet and ensure that elements such as manganese and nitrogen are fully dissolved. The heating temperature is 1200℃ and the time is 3h.

[0087] Step 4, hot rolling:

[0088] The homogenized forging blank is subjected to hot rolling, with an initial rolling temperature of 1080℃ and a final rolling temperature of 850℃, which is lower than the austenite recrystallization temperature, to ensure that most of the deformed austenite grains are retained after rolling. The reduction rate of each pass is not greater than 30% to avoid cracks caused by excessive deformation in a single pass. Seven passes are adopted, with a total reduction rate of 88%.

[0089] Step 5, cooling:

[0090] The hot-rolled plate is water-cooled to room temperature to obtain the high-strength high-plasticity steel for ultra-low temperature environments.

[0091] Figure 1 EBSD map of the microstructure of the high-strength high-plasticity steel for ultra-low temperature environments prepared in Example 1. From Figure 1 As can be seen, the high-strength high-plasticity steel prepared in this example is a single-phase austenite structure, and the specific structure is about 90% deformed austenite grains and 10% equiaxed austenite grains, as counted by an EBSD data analysis software.

[0092] The deformed austenite grains have a high dislocation density, which produces a significant dislocation strengthening effect, thereby effectively improving the strength of the material, but stress concentration is easily produced, which reduces the elongation; the equiaxed austenite grains contain a large number of twins, which can increase the average free path of dislocation movement and coordinate the deformation between different grains to avoid crack generation and improve the plasticity of the material, but the dislocation density of the equiaxed austenite grains is low, which will sacrifice part of the strength of the material.

[0093] After detection, the high-strength high-plasticity steel for ultra-low temperature environments prepared in this example has the following mechanical properties at room temperature, -196℃ and -269℃:

[0094]

[0095] The high-strength high-plasticity steel for ultra-low temperature environments in this example has a magnetic permeability of 1.022 when the external magnetic field strength is 5T and the test temperature is -269℃.

[0096] Example 2

[0097] The difference between this example and Example 1 is that the hot-rolled plate is air-cooled to room temperature and then placed in a box-type resistance furnace for offline solid solution at 880℃ for 10 minutes, and then water-cooled to room temperature. The rest of the preparation method is exactly the same as that of Example 1, and will not be repeated here.

[0098] Figure 2 EBSD map of the microstructure of the high-strength high-plasticity steel for ultra-low temperature environments prepared in Example 2. FromFigure 2 It can be seen that, compared with example 1, more equiaxed austenite grains appear, and the number of deformed austenite grains decreases. The specific organization is about 80% deformed austenite grains and 20% equiaxed austenite grains by a kind of EBSD data analysis software statistics.

[0099] It is detected that the high-strength high-plasticity steel prepared in the embodiment has the following mechanical properties at room temperature, -196 DEG C and -269 DEG C:

[0100]

[0101] Compared with example 1, the elongation of the high-strength high-plasticity steel prepared in the embodiment at each temperature is slightly improved, but the yield strength and tensile strength decrease. The main reason is that the proportion of deformed austenite grains and equiaxed austenite grains changes, the volume fraction of equiaxed austenite grains increases, and the volume fraction of deformed austenite grains decreases.

[0102] In the embodiment, the high-strength high-plasticity steel for ultra-low temperature environment has a magnetic permeability of 1.019 when the external magnetic field strength is 5T and the test temperature is -269 DEG C.

[0103] Example 3

[0104] The difference between the embodiment and example 2 is that the hot-rolled plate is air-cooled to room temperature, then placed in a box-type resistance furnace for offline solid solution at 950 DEG C for 10 min, and then water-cooled to room temperature. The rest of the preparation method is exactly the same as that of example 2, and will not be repeated here.

[0105] Figure 3 The EBSD diagram of the microstructure of the high-strength high-plasticity steel for ultra-low temperature environment prepared in example 3. From Figure 3 It can be seen that, compared with example 2, more equiaxed austenite grains appear, and the number of deformed austenite grains decreases. The specific organization is about 30% deformed austenite grains and 70% equiaxed austenite grains by a kind of EBSD data analysis software statistics.

[0106] It is detected that the high-strength high-plasticity steel prepared in the embodiment has the following mechanical properties at room temperature, -196 DEG C and -269 DEG C:

[0107]

[0108] Compared with Example 2, the high-strength high-plasticity steel for ultra-low temperature environment prepared in the embodiment has further improved elongation at various temperatures, but the yield strength and tensile strength decrease. The main reason is that the proportion of deformed austenite grains and equiaxed austenite grains changes, the volume fraction of equiaxed austenite grains increases, and the volume fraction of deformed austenite grains decreases.

[0109] In the embodiment, the high-strength high-plasticity steel for ultra-low temperature environment has a magnetic permeability of 1.015 when the external magnetic field strength is 5T and the test temperature is -269℃.

[0110] Example 4

[0111] The difference between the embodiment and Example 1 is that the composition of the electroslag remelted ingot is different, the actual measurement is shown in the following table (wt%, S=0.003, P=0.003, Fe balance), the finish rolling temperature is different, which is 880℃, and the rest of the preparation method is the same as Example 1, which will not be repeated here.

[0112]

[0113] Figure 4 The EBSD microstructure diagram of the high-strength high-plasticity steel for ultra-low temperature environment prepared in Example 4. From the Figure 4 It can be seen that, compared with Example 1, the proportion of deformed austenite grains in the embodiment decreases slightly, and through a kind of EBSD data analysis software statistics, the specific organization is about 75% of deformed austenite grains and 25% of equiaxed austenite grains.

[0114] After detection, the mechanical properties of the high-strength high-plasticity steel for ultra-low temperature environment prepared in the embodiment at room temperature, -196℃ and -269℃ are as follows:

[0115]

[0116] Compared with Example 1, the high-strength high-plasticity steel for ultra-low temperature environment prepared in the embodiment has small decreases in yield strength, tensile strength and elongation at various temperatures. The main reason is that the mass percentages of Mn, N and Cr elements increase compared with Example 1, the finish rolling temperature increases, the volume fraction of equiaxed austenite grains increases, and the volume fraction of deformed austenite grains decreases.

[0117] In the embodiment, the high-strength high-plasticity steel for ultra-low temperature environment has a magnetic permeability of 1.024 when the external magnetic field strength is 5T and the test temperature is -269℃.

[0118] Example 5

[0119] The difference between the present example and Example 1 is that the composition of the electroslag remelted ingot is different, and the actual measurement is shown in the following table (wt%, S = 0.003, P = 0.003, Fe balance), the finish rolling temperature is different, which is 820℃, and the rest of the preparation method is exactly the same as that of Example 1, and will not be repeated here.

[0120]

[0121] Figure 5 The EBSD map of the microstructure of the high-strength and high-plasticity steel for ultra-low temperature environment prepared in Example 5 is shown in Figure 5. Figure 5 It can be seen that, compared with Example 1, the proportion of equiaxed austenite grains in the present example is slightly more than that in Example 1, and the specific organization is about 85% deformed austenite grains and 15% equiaxed austenite grains by statistical analysis of an EBSD data analysis software.

[0122] It is detected that the mechanical properties of the high-strength and high-plasticity steel for ultra-low temperature environment prepared in the present example at room temperature, -196℃ and -269℃ are as follows:

[0123]

[0124] Compared with Example 1, the yield strength and tensile strength of the high-strength and high-plasticity steel for ultra-low temperature environment prepared in the present example at each temperature have a small decrease, and the elongation is relatively close. The main reason is that the mass percentages of Mn element, Cr element and N element are decreased compared with Example 1, and the finish rolling temperature is decreased.

[0125] The permeability of the high-strength and high-plasticity steel for ultra-low temperature environment in the present example is 1.019 when the external magnetic field strength is 5T and the test temperature is -269℃.

[0126] Comparative Example 1

[0127] The difference between the present comparative example and Example 1 is that the finish rolling temperature of hot rolling is 1040℃, and the rest of the preparation method is exactly the same as that of Example 1, and will not be repeated here.

[0128] Figure 6 The EBSD map of the microstructure of the material prepared in Comparative Example 1 is shown in Figure 6. Figure 4 It can be seen that, compared with Example 1, the present comparative example is completely composed of equiaxed austenite grains, without deformed austenite grains.

[0129] It is detected that the mechanical properties of the material prepared in the present comparative example at room temperature, -196℃ and -269℃ are as follows:

[0130]

[0131] Compared with Example 1, the yield strength and tensile strength of the material prepared in the present comparative example at each temperature are greatly reduced, and the yield strength at -269℃ is <1500MPa. The main reason is that the finish rolling temperature exceeds the limited 820℃-880℃, the microstructure is completely composed of equiaxed austenite grains, there is no deformed austenite grain, and the volume fraction of deformed austenite grain is lower than the limited 30%-90%, resulting in a decrease in strength.

[0132] Comparative Example 2

[0133] The difference between the present comparative example and Example 2 is that the hot-rolled plate is air-cooled to room temperature, then placed in a box-type resistance furnace for offline solid solution at 1000℃ for 10min, and then water-cooled to room temperature. The rest of the preparation method is exactly the same as that of Example 2, and will not be repeated here.

[0134] Figure 7 The microstructure EBSD map of the material prepared in Comparative Example 2 is shown in Figure 2. Figure 7 As can be seen, compared with Example 2, the deformed austenite grains of the present comparative example are greatly reduced, and the specific microstructure is about 10% deformed austenite grains and 90% equiaxed austenite grains, which is statistically analyzed by an EBSD data analysis software.

[0135] After detection, the mechanical properties of the material prepared in the present comparative example at room temperature, -196℃ and -269℃ are as follows:

[0136]

[0137] Compared with Example 2, the yield strength and tensile strength of the material prepared in the present comparative example at each temperature are greatly reduced, and the yield strength at -269℃ is <1500MPa. The main reason is that the volume fraction of deformed austenite grain is lower than the limited 30%-90%, resulting in a decrease in strength.

[0138] Comparative Example 3

[0139] The difference between the present comparative example and Example 1 is that the composition of the electroslag remelted ingot is different, and the actual measurement is shown in the following table (wt%, S=0.004, P=0.002, Fe balance), and the rest of the preparation method is exactly the same as that of Example 1, and will not be repeated here.

[0140]

[0141] Figure 8 The microstructure EBSD map of the material prepared in Comparative Example 3 is shown in Figure 4. Figure 8 As can be seen, compared with Example 1, the proportion of deformed austenite grains of the present comparative example is further improved, and the specific microstructure is about 95% deformed austenite grains and 5% equiaxed austenite grains, which is statistically analyzed by an EBSD data analysis software.

[0142] The mechanical properties of the material prepared in the present comparative example at room temperature, -196°C and -269°C were as follows:

[0143]

[0144] Compared with Example 1, the yield strength, tensile strength and elongation of the material prepared in the present comparative example at each temperature were all greatly decreased, and brittle fracture occurred in the tensile test at -196°C and -269°C, with elongation of only 6% and 4%, respectively. The main reason was that the mass percentage of Mn element was decreased compared with Example 1, which was lower than the limited range of 20.0% to 28.0%, and did not meet the requirement of and , the stability of austenite was poor, and the volume fraction of deformed austenite grains exceeded the limited range of 30% to 90%, which led to brittle fracture due to rapid phase transformation of a large amount of martensite at low temperature.

[0145] Comparative Example 4

[0146] The present comparative example was different from Example 1 in that the composition of the electroslag remelted ingot was different, and the actual measurement was as shown in the following table (wt%, S = 0.003, P = 0.001, Fe balance), and the rest of the preparation method was the same as Example 1, which will not be repeated here.

[0147]

[0148] Figure 9 EBSD diagram of the microstructure of the material prepared in Comparative Example 4. From Figure 9 it can be seen that, compared with Example 1, the proportions of deformed austenite grains and equiaxed austenite grains in the present comparative example were relatively close, and the specific structure was about 87% deformed austenite grains and 13% equiaxed austenite grains by statistical analysis of an EBSD data analysis software.

[0149] The mechanical properties of the material prepared in the present comparative example at room temperature, -196°C and -269°C were as follows:

[0150]

[0151] Compared with Example 1, the yield strength and tensile strength of the material prepared in the present comparative example at each temperature were both significantly decreased, and the yield strength at -269°C was less than 1200 MPa. The main reason was that the mass percentage of N element was decreased compared with Example 1, which was lower than the limited range of 0.25% to 0.40%, and did not meet the requirement of , the solid solution strengthening effect of the material was sharply decreased, and even if the volume fraction of deformed austenite grains was within the limited range, the strength was still greatly lost.

[0152] The above has shown and described the embodiments of the present application, it can be understood that the above embodiments are exemplary and cannot be understood as limiting the present application, the ordinary skilled in the art can modify the technical solutions recorded in the foregoing embodiments or make equivalent replacement to part of the technical features within the scope of the present application; and the modification or replacement does not make the essence of the corresponding technical solution deviate from the scope of the present application.

Claims

1. A high-strength high-plasticity steel for use in an ultralow-temperature environment, characterized by, The chemical composition of the high-strength and high-plasticity steel for ultra-low temperature environment, in mass percentage, comprises: C≤0.03%; Mn: 20.0%-28.0%; Cr: 10.0%-15.0%; Si: 0.20%-0.40%; N: 0.25%-0.40%; Cu≤1.0%; Nb≤0.20%; S≤0.005%; P≤0.005%; the balance being Fe and inevitable impurities; the mass percentages of Mn, Cr, N and Si satisfy the following relations: (Mn+25N) / (Cr+2Si)≥1.66; 0.21(Cr+2Si)+Mn+25N≥28.

4. V≤0.20%； The high-strength and high-plasticity steel for ultra-low temperature environment has a stable single-phase austenite structure, and no martensite phase is generated at -269℃ detected by X-ray diffraction or EBSD analysis; the volume fraction of deformed austenite grains in the matrix structure of the high-strength and high-plasticity steel for ultra-low temperature environment is≥30% and≤90%; the grains with a local orientation difference greater than 0.65° are defined as deformed austenite grains; The preparation method of the high-strength and high-plasticity steel for ultra-low temperature environment comprises the following steps: Step 1, smelting: smelting according to the composition of the high-strength and high-plasticity steel for ultra-low temperature environment to obtain an alloy ingot, and ensuring that the nitrogen content in the alloy ingot after smelting is 0.25wt.%-0.40wt.%; Step 2, forging: performing forging treatment on the alloy ingot to obtain a forged blank; the heating temperature of forging is 1070℃-1150℃, and the final forging temperature is 950℃-1030℃; Step 3, heating: performing homogenization treatment on the forged blank before hot rolling to ensure that the manganese and nitrogen elements are fully solid-solved, and the heating temperature is 1200℃-1250℃ and the time is 2-4h; Step 4, hot rolling: performing hot rolling on the homogenization-treated forged blank to obtain a plate; the opening rolling temperature is 1050℃-1150℃, the final rolling temperature is 820℃-880℃, and the single-pass reduction rate of hot rolling is not greater than 30%; Step 5, cooling: water cooling or air cooling the hot-rolled plate to room temperature to obtain the high-strength and high-plasticity steel for ultra-low temperature environment; after air cooling to room temperature, offline solid solution treatment is performed at 880℃-950℃ for 5-15min. The relative magnetic permeability of the high-strength and high-plasticity steel for ultra-low temperature environment is<1.03 under a 5T magnetic field strength at -269℃. The yield strength of the high-strength and high-plasticity steel for ultra-low temperature environment is>550MPa, the tensile strength is>800MPa, and the elongation is>35% under room temperature tensile test; 2. The high-strength high-plasticity steel for ultra-low temperature environments according to claim 1, characterized by The yield strength of the high-strength and high-plasticity steel for ultra-low temperature environment is>1250MPa, the tensile strength is>1600MPa, and the elongation is>35% under -196℃ tensile test; 3. The high-strength high-plasticity steel for ultra-low temperature environments according to claim 1, characterized by The yield strength of the high-strength and high-plasticity steel for ultra-low temperature environment is>1600MPa, the tensile strength is>1900MPa, and the elongation is>20% under -269℃ tensile test. The smelting method is vacuum consumable or electroslag remelting: The vacuum consumable reduces the escape of nitrogen elements due to the increase of vacuum degree through argon partial pressure control, and ensures that the nitrogen yield is≥80%.

4. The high-strength high-plasticity steel for ultra-low temperature environments according to claim 1, characterized by ​ ​ The electroslag remelting adopts CaF2-CaO-Al2O3-MgO quaternary slag system, and is carried out under a dry air protective atmosphere.

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