A nano-austenite skeleton toughened high plasticity steel and a preparation method thereof

Abstract

The application discloses a new design idea of high-ductility structural steel, and particularly relates to a nano-austenite skeleton toughened high-plasticity steel and a preparation method thereof. The application breaks through the conventional idea of adding a large amount of alloy to obtain a high proportion of austenite to realize high plasticity, and realizes a significant improvement in plasticity by only 15vol% of austenite. The application breaks the traditional cognition that solidification segregation deteriorates material performance, and skillfully utilizes the Mn micro-segregation structure under non-equilibrium solidification, combines the ingenious control of rolling and tempering with appropriate parameters, realizes effective regulation of the distribution and density characteristics of the austenite stable elements Mn and C, and further realizes global regulation of the austenite. Through the synergistic effect of the austenite skeleton effect and the TRIP effect, the application maximizes the contribution of austenite to toughness and plasticity, so that the steel has an ultrahigh ductility of >30% under the condition of low carbon and low alloy element addition, and simultaneously has an ultimate tensile strength of >1.0 GPa.

Description

[0001] TECHNICAL FIELD

[0002] The application belongs to the technical field of structural steel design and superplastic forming, and particularly relates to a nano-austenite skeleton toughened high-plasticity steel and a preparation method thereof. BACKGROUND

[0003] In response to global climate change, reducing carbon emissions and promoting sustainable development, in this context, the steel industry as an important energy consumption and carbon emission industry, to strengthen the research and development of new technologies, to promote high-quality, sustainable development of advanced steel materials is of great significance. Lightweight high-quality steel with high ductility is a high-quality structural material, which plays an irreplaceable role in reducing carbon emissions, improving energy utilization and infrastructure safety.

[0004] A substantial increase in strength often leads to a significant decrease in ductility, i.e. the so-called "strength-ductility" constraint balance. In order to obtain high ductility, a large amount of austenite stabilizing elements (including titanium, chromium, manganese, nickel, cobalt and other noble metal elements) are usually added to obtain a large amount of austenite structure at room temperature, so as to continuously occur TRIP effect and / or TWIP effect during deformation to obtain excellent ductility. However, the addition of high alloy content not only greatly increases the production cost, but also increases the metallurgical difficulty, which greatly restricts the industrialization and popularization of advanced high-quality steel. In addition, the realization of excellent comprehensive mechanical properties of advanced high-quality steel usually requires the synergistic use of multiple strengthening and toughening mechanisms (fine-grain strengthening, phase transformation strengthening, dislocation strengthening, solid solution strengthening, grain boundary strengthening, etc.) to achieve the target strength and plasticity, which makes the production process long and complex (repeated rolling / forging, multi-stage annealing, deep cryogenic treatment, etc.), and becomes another important reason for restricting its large-scale production and popular application.

[0005] For example: the patent "High-strength steel with excellent ductility and its manufacturing method" (publication number CN 112760554A, publication date 2021.05.07) discloses a high-strength steel manufacturing method with yield strength of 550-850 MPa, tensile strength of 900-1100 MPa, uniform elongation ≥13%, and fracture elongation of 18%-28%. To obtain this performance, the technical solution provided is: the mass percentage of chemical elements is: C: 0.15-0.25wt%, Si: 1.00-2.00wt%, Mn: 1.50-3.00wt%, Al: 0.03-0.06wt%, and the balance is Fe and other unavoidable impurities; the manufacturing method includes the following steps: (1) smelting and thin slab continuous casting: the slab thickness at the outlet end of the continuous casting is controlled to be 55-60mm; (2) heating: heat the slab to 1200-1250℃; (3) hot rolling: control the final rolling temperature to be 860-930℃, and the coiling temperature to be 450-600℃, the thickness of the oxide scale on the surface of the steel strip after hot rolling is ≤5μm, and the (FeO+Fe3O4) in the oxide scale on the surface of the steel strip after hot rolling is ≤40wt%; (4) pickling or pickling+cold rolling: when pickling+cold rolling is used, the deformation amount is controlled to be 30%-70%; (5) continuous annealing: control the continuous annealing process to meet at least one of the following: annealing temperature 820-870℃; slow cooling to 700-730℃ at a cooling rate of 3-10℃ / s; fast cooling to 280-320℃; after fast cooling, heat to 400-430℃ and hold for 180-300s; control the volume content of hydrogen in the reducing atmosphere in the continuous annealing furnace to be 10-15%. In the technical solution disclosed in this patent, the heating temperature and the annealing temperature are both high, the energy consumption is large, and the process flow is long, the operation process is complex, the operation precision requirement is high, which is not conducive to large-scale production.

[0006] For example, the patent "A low-density ultra-high-strength high-plasticity steel and its preparation method and application" (publication number CN114752864 A, publication date July 15, 2022) discloses a preparation method of high-manganese TWIP steel with yield strength ≥ 950 MPa, tensile strength ≥ 1050 MPa, and elongation ≥ 30%. The manufacturing method to obtain this performance is: the element content (by mass percentage) includes Mn: 30-34%, Al: 11-11.9%, C: 1.2-1.29%, Cr: 4-7%, Cu: 0.5-1.2%, Nb: 0.01-0.3%, V: 0.01-0.3%, Ti0:.01-0.3%, La: 0.05-0.1%, B: 0.0001-0.005%, N: 0.05-0.1%, P ≤ 0.012%, S ≤ 0.003%, the balance of iron and unavoidable impurities; the production steps include: smelting and casting to obtain ingots, forging forming, rolling, quenching and solid solution, and low-temperature aging treatment. The process of forging forming is: heating to 1100-1140℃ at a rate of 15-20℃ / h, holding for ≥ 10h, then forging according to the processes of shaping, broadening, lengthening and shaping, and whenever the forged piece cools to 950℃, heating to 1100-1140℃ and holding for ≥ 1h. The process of rolling is: heating to 1140-1180℃ at a rate of 20-30℃ / h, holding after heating, and then rolling out of the furnace, with a rolling temperature of 1120-1140℃ and a pass reduction of 6-20mm, and a final rolling temperature ≥ 950℃. The cooling rate of quenching and solid solution is ≥ 30℃ / s. The aging treatment temperature is 450-550℃, and the time is 3-6h. It should be noted that in the technical solution disclosed in this patent, a very high proportion of alloying elements are added to obtain an austenitic matrix (≥ 94vol%) + a small amount of δ ferrite and (Nb, V, Ti)(C, N) precipitated phase organization, which is extremely high in cost, long in process flow, complex in operation process, and difficult to realize industrialization.

[0007] For another example, the patent "1180MPa grade precipitation strengthening type high strength high plasticity steel and its manufacturing method" (authorization number CN 106119700 B, authorization date 2018.06.01) realizes a relatively excellent mechanical property combination of yield strength ≥1000MPa, tensile strength ≥1180MPa, and elongation ≥15% (in the examples of the patent, the maximum elongation of the product is only 18%). It needs to be pointed out that in the technical solution disclosed in the patent, the heating temperature of the casting blank / ingot is high and the heating time is long (the heating temperature is ≥1230℃, and the heating time is 1-2 hours), and a complex process flow of multi-pass hot rolling (3-5 passes of rough rolling, deformation ≥50%, 3-5 passes of finish rolling, deformation ≥80%) + segmented cooling + coiling control is needed to finally obtain a relatively small (≤5μm) ferrite and bainite structure to ensure the elongation; in addition, Ti, Nb, V and other noble metals are added to obtain nanoscale carbides to realize precipitation strengthening to ensure high strength.

[0008] In summary, the existing advanced high-strength steel has the objective problems of high carbon, high alloy, and high cost, and faces the development bottleneck of long and complex production process and great metallurgical difficulty, which is not conducive to the industrialization of advanced high-quality steel and the realization of the "double carbon target". Therefore, it is of great significance for the sustainable development of China's steel industry to design and develop a high-quality steel with low alloy, short process, strong universality, and high ductility. SUMMARY

[0009] The purpose of the present application is to design and develop a high-quality steel with low alloy, short process, strong universality, and high ductility, and to expand the design dimension of high-ductility structural materials, which is conducive to their industrialization and popularization.

[0010] In order to achieve the above-mentioned purpose, the present patent proposes a nano-austenite skeleton toughening type high plasticity steel and its preparation method.

[0011] The nano-austenite skeleton toughening type high plasticity steel of the present application, the total volume fraction of austenite in the whole structure is about 7-15vol.%, and the austenite includes nano-austenite and sub-micron size thin film / block austenite.

[0012] The nano-austenite skeleton toughening type high plasticity steel of the present application, the nano-austenite skeleton structure coordinates deformation through skeleton effect; the remaining sub-micron size thin film / block austenite further improves the strength and plasticity of the material through TRIP effect; the synergistic effect of skeleton effect and TRIP effect realizes the improvement of ductility without reducing strength.

[0013] The application does not need to introduce noble metals such as Nb, V and rare earth, uses nano-austenite as a body skeleton structure, coordinates deformation through skeleton effect, and further improves the strength and plasticity of the material through TRIP effect of the rest sub-micron size thin film / block austenite; and the synergistic effect of the skeleton effect and the TRIP effect realizes the improvement of the ductility of the material without reducing the strength.

[0014] The application discloses a preparation method of a nano-austenite skeleton toughened high plasticity steel.

[0015] The steel liquid is composed of the following components in percentage by mass:

[0016] C: 0.15-0.22%, Si: 1.3-2.0%, Mn: 1.6-2.2%, Al≤0.003%, Cr≤0.05%, Ti≤0.003%, P:≤0.007%, S≤0.002%, and the rest is Fe and inevitable impurities.

[0017] As a preferred solution, the chemical composition of the steel liquid of the preparation method of the nano-austenite skeleton toughened high plasticity steel is as follows: C: 0.165-0.175%, Si: 1.85-2.05%, Mn: 1.75-1.85%, Cr: 0.02-0.04%, Al: 0.001-0.003%, Ti: 0.001-0.0025%, P:≤0.007%, S≤0.002%, and the rest is Fe and inevitable impurities.

[0018] As a preferred solution, the preparation method of the nano-austenite skeleton toughened high plasticity steel has a sub-rapid solidification technology cooling speed of 100-2000k / s, and further preferably 1000-2000k / s.

[0019] As a preferred solution, the preparation method of the nano-austenite skeleton toughened high plasticity steel has an isothermal short-time annealing temperature of 30-100°C higher than the Ac3 temperature, and a time of 180-600s.

[0020] As a further preferred embodiment, the present invention provides a method for preparing a nano-austenitic skeleton-toughened high-ductility steel, wherein the full austenitization temperature range is 30–80°C above the Ac3 temperature, and the isothermal annealing time is 180–480 s. As an even more preferred embodiment, the full austenitization temperature is 70–80°C above the Ac3 temperature, and the annealing time is 250–300 s. In this invention, Ac3 is the final temperature at which all ferrite transforms into austenite.

[0021] As a further preferred embodiment, the present invention provides a method for preparing a nano-austenitic skeleton toughened high-ductility steel, wherein the annealing temperature is 935–965°C.

[0022] As a preferred embodiment, the present invention provides a method for preparing a nano-austenitic skeleton toughened high-ductility steel, wherein the hot rolling temperature range is 800–950°C. More preferably, it is 830–940°C; even more preferably, the initial rolling temperature is 900–940°C, the final rolling temperature is 830–870°C, and the reduction rate is 20–60%.

[0023] As a further preferred embodiment, the present invention provides a method for preparing a nano-austenitic skeleton toughened high-plasticity steel, wherein the hot rolling reduction rate is 28-55%, and more preferably 43-47% or 28-35%.

[0024] As a preferred embodiment, the present invention provides a method for preparing a nano-austenitic skeleton toughened high-plasticity steel, wherein the hot-rolled steel strip is air-cooled to a temperature range of 55-95°C below the martensite initiation transformation temperature Ms, and then isothermally tempered for a short time at this temperature for 300-600s.

[0025] As a further preferred embodiment, the present invention provides a method for preparing a nano-austenitic skeleton toughened high-plasticity steel, wherein the rolled steel strip is air-cooled to a temperature range of 80-90°C below the martensite initiation transformation temperature Ms, and held at this temperature for 300s.

[0026] As a preferred embodiment, the present invention provides a method for preparing a nano-austenitic skeleton toughened high-ductility steel, wherein the steel sample is immediately water-quenched after tempering.

[0027] As a further preferred option, the tempering temperature is preferably 285–315°C.

[0028] As a preferred embodiment, the present invention provides a method for preparing a nano-austenitic skeleton toughened high-ductility steel, wherein the microstructure of the resulting thin strip steel is as follows: nano-austenite is uniformly dispersed at the refined lath martensite grain boundaries / twin boundaries, forming a strong austenitic skeleton structure that runs through the entire material; a small amount of submicron-sized thin film / bulk austenite is uniformly distributed between the martensite laths.

[0029] Specifically:

[0030] Spherical / near-spherical nano-austenite is uniformly dispersed at the grain boundaries and / or twin boundaries of refined lath martensite, forming a strong austenitic framework structure that runs throughout the entire material. Submicron-sized thin films and / or bulk austenite are uniformly distributed between the martensite laths. The nano-austenite, acting as the framework structure, is spherical / near-spherical and distributed near grain boundaries and / or twin boundaries, forming barriers of a certain number, size, and shape in the matrix. This makes the steel texture more complex, maximizing the role of the framework effect in coordinating deformation and achieving the maximum improvement in the toughness and plasticity of the material with minimal austenite. Thin films and / or bulk austenite further enhance the strength and plasticity of the material through the TRIP effect during stress-strain processes.

[0031] In the product obtained by this invention, the total volume fraction of austenite in the entire microstructure is approximately 7-15 vol.%, preferably 8-10 vol.%. The nano-austenitic framework structure coordinates deformation through the framework effect, maximizing the material's toughness and plasticity with minimal austenite. The remaining submicron-sized thin films and / or bulk austenite further enhance the material's strength and plasticity through the TRIP effect. The synergistic effect of the framework and TRIP effects significantly improves the material's ductility without reducing strength.

[0032] The present invention, without the addition of rare earth elements and V, produces strip steel with a yield strength of 675-950 MPa, tensile strength >1000 MPa, elongation after fracture ≥30%, and strength-ductility product >30 GPa through a simple preparation process.

[0033] After optimization, this invention provides a method for preparing a nano-austenitic skeleton-toughened high-ductility steel. The synergistic effect of the skeleton effect and the TRIP effect enables the steel to exhibit ultra-high ductility with low carbon and low alloy element additions, while also possessing high yield strength and ultimate tensile strength. The resulting strip steel has a yield strength of 850–950 MPa, a tensile strength >1000 MPa, an elongation after fracture ≥30%, preferably 30–40%, and a strength-ductility product >30 GPa%, preferably 30–40 GPa%.

[0034] In this invention, after tempering, the steel sample is immediately subjected to water quenching to obtain the target steel strip.

[0035] As a better demonstration of the synergistic effect of composition and process, the present invention provides a method for preparing a nano-austenitic skeleton toughened high-ductility steel; the chemical composition of the molten steel cast in this method is as follows (mass percentage): C: 0.165-0.175%, Si: 1.85-2.05%, Mn: 1.75-1.85%, Cr: 0.02-0.04%, Al: 0.001-0.003%, Ti: 0.001-0.0025%, P: ≤0.007%, S≤0.002%, with the remainder being Fe and unavoidable impurities, and the cooling rate of the sub-rapid solidification technology is controlled at 1000-2000 k / s;

[0036] When the isothermal short-time annealing temperature is controlled at 935–965℃ and the time at 300s; and the hot rolling initial rolling temperature is controlled at 900–940℃, the final rolling temperature at 830–870℃, the reduction rate at 43–47%, and the tempering temperature at 285–315℃ and the tempering time at 300s, the resulting strip steel has a yield strength greater than or equal to 864MPa, a tensile strength greater than or equal to 1075MPa, an elongation after fracture ≥35%, and a strength-ductility product of approximately 39GPa.

[0037] In the composition design of the steel of this invention:

[0038] Carbon (C), as an interstitial atom, is crucial for improving the strength of steel and is an indispensable strengthening element in high-strength steel. Simultaneously, C is the most fundamental element for stabilizing austenite. In this invention, to obtain the target strength and plasticity, it is necessary to obtain austenite of suitable quantity, size, and stability, as well as a martensitic matrix with suitable strength and plasticity at room temperature. Therefore, the carbon content in the steel must be above 0.15 wt.%. However, the C content must be within a suitable range to maximize its role in stabilizing retained austenite. Excessive C content will promote the precipitation of carbides, thereby reducing the proportion of C stabilizing retained austenite in the microstructure, ultimately leading to the loss of retained austenite.

[0039] Insufficient stability. Furthermore, excessively high carbon content is detrimental to the weldability of the steel and the achievement of carbon reduction targets. Therefore, the C content in this invention is 0.15–0.22 wt.%, preferably 0.16–0.18 wt.

[0040] Manganese (Mn) is a strong austenite stabilizing element. By improving the stability of austenite, a portion of austenite is retained at room temperature. This invention utilizes the characteristics of Mn in stabilizing austenite and its low diffusion coefficient and slow diffusion, making it prone to forming micro-segregation at grain boundaries under sub-rapid solidification conditions. In this patent, the microstructure with Mn microsegregation obtained by sub-rapid solidification is cleverly utilized first. Then, by combining the skillful control of the rolling and annealing processes, the micro-segregated structure of Mn obtained by sub-rapid solidification is modified and inherited, thereby achieving global control over the austenite structure in the final microstructure. This invention innovatively proposes the austenite skeleton effect—forming a fine, dispersed nano-austenite skeleton at grain boundary locations in the matrix to maximize its contribution to toughness and plasticity. Therefore, controlling the size, morphology, and distribution of austenite is crucial, without necessarily obtaining a high proportion of austenite. Therefore, the Mn content in this invention is controlled at 1.6–2.2 wt.%, and its distribution density can be adjusted by regulating its content.

[0041] In this invention, Si has very low solubility in carbides, which strongly inhibits the formation of carbide precipitates, reduces C loss, and allows C to accumulate as much as possible in austenite, thereby increasing the austenite ratio. However, excessive Si content will severely deteriorate the surface quality and processing performance of the steel. Therefore, the Si content in this invention is 1.3–2.0%, preferably 1.7–1.9%.

[0042] The addition of other elements is mainly based on the actual situation and the required performance, and is either a well-known technology in the field or exists as impurity elements.

[0043] This invention discloses a nano-austenitic skeleton toughened high-ductility steel and its preparation method, which has the following advantages compared with conventional advanced high-strength steel:

[0044] 1. Technological and cost advantages

[0045] Low alloy and low cost: By using only a small amount of carbon and manganese, it breaks through the conventional design approach of obtaining a high proportion of austenite through the addition of a large amount of alloying elements to achieve excellent strength and plasticity through continuous phase transformation (TRIP effect), thus expanding the design dimensions of advanced high-quality steel; Short process, energy saving and carbon reduction: The heat treatment process is cleverly coupled to the hot rolling and coiling processes of the production line, which greatly shortens the production process and makes full use of the heat in the hot rolling and coiling processes, avoiding energy waste.

[0046] 2. Organizational advantages

[0047] Breaking away from the traditional understanding that solidification segregation degrades material properties, this study cleverly utilizes a micro-segregated structure with Mn obtained through sub-rapid solidification, combined with precise control of rolling and tempering, to achieve global regulation of the distribution and density characteristics of the austenite-stabilizing elements Mn and C. This allows for effective control of the austenite microstructure in the final microstructure, resulting in the design of a nano-austenite framework structures. The austenite in this framework structure is small in size and uniformly dispersed at the grain boundaries / subgrain boundaries of the refined matrix phase, which helps to restrict microstructural deformation during stretching. This restriction hinders dislocation slip, thereby stabilizing material deformation and, to some extent, offsetting the effects of work hardening, resulting in better plasticity and ductility. Furthermore, compared to other forms of austenite, the austenite in the framework structure exhibits higher stability due to its size and distribution characteristics.

[0048] 3. Performance advantages

[0049] This invention discloses a nano-austenitic skeleton-toughened high-plasticity steel and its preparation method. The product obtained by the method comprehensively utilizes multiple strengthening and toughening mechanisms, and its mechanical properties are comparable to those of medium-manganese steel. By utilizing the skeleton effect in conjunction with the TRIP effect, the contribution of austenite to toughness and plasticity is maximized, and excellent strength and plasticity are achieved under the condition of low austenite content. The TRIP effect coupled with fine grain strengthening ensures the high strength of the material. ZYLiang et al. studied medium-manganese steel with a composition of Fe-7Mn-0.14C-0.23Si (wt.%), and the optimal mechanical properties obtained were an ultimate tensile strength of 1003 MPa and an elongation of 24.4% (ZYLiang, ZHCao, J.Lu, MXHuang, CCTasan, Influence of co-existing mediumMn and dual-phase steel microstructures on ductility and Lüders bandformation, ActaMaterialia, Volume 221, 2021, 117418). Ran Ding et al. studied the microstructure and mechanical properties of Fe-0.2C-8Mn-2Al (wt.%) medium-manganese steel using traditional continuous casting technology combined with Q-ART process, and finally obtained the optimal mechanical property combination of an ultimate tensile strength of 950 MPa and an elongation of 34.5% and an ultimate tensile strength of 1135 MPa and an elongation of 30.3% (Ran Ding, Zongbiao Dai, Mingxin Huang, Zhigang). Yang, Chi Zhang, Hao Chen, Effect of pre-existed austenite on austenite reversion and mechanical behavior of anFe-0.2C-8Mn-2Al medium Mn steel, ActaMaterialia, Volume 147, 2018, Pages 59-69). Attached Figure Description

[0050] Figure 1 The results of electron probe microanalysis (EPMA) of manganese in the cast strip of Example 3 are shown.

[0051] Figure 2 The results of electron probe microanalysis (EPMA) of carbon elements in the cast strip of Example 3 are shown.

[0052] Figure 3 The results of electron probe microanalysis (EPMA) of manganese in the finished strip steel of Example 3 are shown.

[0053] Figure 4The results of electron probe microanalysis (EPMA) of carbon element in the finished steel strip of Example 3 are shown.

[0054] Figure 5 This is a superimposed image of the electron backscatter diffraction (EBSD) FCC color phase diagram and image quality (BC) diagram of the surface and cross-section of the finished steel strip sample in Example 3, where green represents the FCC phase and gray represents the BCC phase.

[0055] Figure 6 This is a geometrically required dislocation (GND) density map of the surface and cross-section of the finished steel strip sample in Example 3;

[0056] Figure 7 The XRD diffraction patterns of the finished steel strip before and after stretching in Example 3 are shown below.

[0057] Figure 8 The stress-strain curves of the finished strip steel in Examples 1-3 are shown.

[0058] from Figure 1 It can be seen that the sample ribbon prepared in Example 3 has typical characteristics: manganese has obvious microsegregation behavior along the original austenite grain boundaries.

[0059] from Figure 2 It can be seen that the sample ribbon prepared in Example 3 has typical characteristics of microstructure: uniform distribution of carbon elements.

[0060] from Figure 3 It can be seen that the finished steel prepared in Example 3 has typical characteristics: manganese has significant microsegregation along the original austenite grain boundaries that refine the grain size. This significant microsegregation becomes a strong barrier to martensite growth during the quenching process, restricting lath martensite to the submicron size and ensuring that the martensitic matrix has high strength and good toughness and plasticity.

[0061] from Figure 4 It can be seen that the finished steel prepared by Example 3 has typical characteristics: carbon elements have obvious micro-agglomeration, each agglomeration region is very small (micrometer / submicrometer range), and the agglomeration points are uniformly dispersed in the microstructure.

[0062] from Figure 5 It can be seen that the typical characteristics of the finished steel structure prepared by Example 3 are as follows: the matrix phase is refined lath martensite (gray), and spherical / near-spherical nano-austenite (green) is uniformly dispersed at the grain boundaries and / or subgrain boundaries of the refined lath martensite, connecting with the grain boundaries to form a network structure, forming a strong austenitic skeleton structure that runs through the entire material; submicron-sized thin film / bulk austenite (green) is uniformly distributed between the martensite laths.

[0063] from Figure 6It can be seen that the finished steel structure prepared by Example 3 has typical characteristics: the matrix has a high dislocation density and the dislocation distribution is quite uniform.

[0064] from Figure 7 The typical characteristics of the microstructure of the finished steel prepared in Example 3 can be observed: both before and after fracture, there are obvious austenite peaks, indicating that the austenite, acting as a skeleton structure, did not undergo phase transformation during strain. This skeleton effect contributes to the material's excellent mechanical properties. Additionally, a portion of the larger / less stable austenite undergoes martensitic transformation, simultaneously improving both the material's strength and plasticity through the TRIP effect. Furthermore, through… Figure 7 The total volume fraction of austenite in the product can be calculated to be approximately 7–15 vol.%.

[0065] from Figure 8 It can be seen that the mechanical properties of the finished steel prepared by Examples 1-3 are as follows: while obtaining excellent total elongation and uniform elongation, the strength is not compromised, indicating that the samples have excellent overall deformation and local deformation capabilities under high stress conditions. Detailed Implementation

[0066] The technical solutions of the embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Embodiments 1-6 and Comparative Examples 1-6 of the present invention are all produced using a method for preparing a nano-austenitic skeleton toughened high-ductility steel and the same. Note that the described embodiments are only some examples of the present invention; therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0067] This invention discloses a nano-austenitic skeleton toughened high-ductility steel and its preparation method, the specific implementation process of which is as follows:

[0068] 1) Steel smelting

[0069] The molten steel obtained from smelting has the following chemical composition by mass percentage: C: 0.17-0.23%, Si: 1.5-2.0%, Mn: 1.6-2.0%, Cr≤0.05%, P:≤0.007%, S≤0.002%, Al≤0.003%, Ti≤0.004%, with the remainder being Fe and unavoidable impurities;

[0070] 2) Sub-rapid solidification technology casting

[0071] The molten steel obtained from smelting is cast into a strip sample using a sub-rapid solidification technology; the sub-rapid solidification cooling rate ranges from 1000 to 2000 k / s.

[0072] 3) Isothermal annealing

[0073] The cast strip sample was heated to the full austenitizing temperature range and then subjected to short-time annealing (885~965℃, 300s);

[0074] 4) Hot rolling

[0075] It is then air-cooled to 800-920℃ and hot-rolled with a reduction rate of 28-52%;

[0076] 5) Isothermal tempering

[0077] The hot-rolled steel strip is air-cooled to a temperature range of 80-90°C below the martensitic transformation temperature Ms, and held at this temperature for 300 seconds.

[0078] 6) Quenching treatment

[0079] The steel strip after tempering can be immediately water-quenched to obtain the finished steel strip.

[0080] Table 1 lists the mass percentages of chemical elements in steel samples of Examples 1-6 and Comparative Examples 1-6.

[0081] Table 1. (wt.% The remainder is Fe and its unavoidable impurities)

[0082]

[0083]

[0084] Table 2 lists the nano-austenitic skeleton toughened high-plasticity steels of Examples 1-6, their preparation methods, and the specific process parameters of Comparative Examples 1-6.

[0085] Table 2.

[0086]

[0087]

[0088] Table 3 lists the mechanical property test results of Comparative Examples 1-6 of the nano-austenitic skeleton toughened high-plasticity steels.

[0089] Table 3.

[0090]

[0091]

[0092] In summary, the examples and comparative results demonstrate that the present invention provides a nano-austenitic skeleton toughened high-ductility steel and its preparation method, offering a design concept and method for lightweight, high-quality structural steel that combines low alloy content, short process, strong versatility, and high ductility. This extends the design dimensions of advanced high-quality steel to the field of carbon steel, and has broad application value and economic benefits.

Claims

1. A nano-austenitic skeleton toughened high-ductility steel, characterized in that: The total volume fraction of austenite in the steel is 7-15 vol.%, and the austenite includes nano-austenite and submicron-sized thin films and / or bulk austenite. The nano-austenitic skeleton toughened high-ductility steel is prepared by the following steps: The molten steel obtained from smelting is cast into a strip using a semi-rapid solidification technology; the strip is heated to the full austenitization temperature range for isothermal short-time annealing, followed by online hot rolling at a temperature of 800~1000℃ and a reduction rate of 20~60%; after rolling, it is air-cooled to a temperature range of 55~95℃ below the martensite initiation transformation temperature Ms, and then short-time isothermal tempered at this temperature for 300~600s, followed by water quenching to obtain the finished steel; the semi-rapid solidification cooling rate is greater than or equal to 100 k / s; the isothermal short-time annealing temperature is 30~100℃ higher than the Ac3 temperature, and the time is 180~600s; The molten steel is composed of the following components by mass percentage: C: 0.15-0.22%, Si: 1.3-2.0%, Mn: 1.6-2.2%, Al≤0.003%, Cr≤0.05%, Ti≤0.003%, P:≤0.007%, S≤0.002%, with the remainder being Fe and unavoidable impurities.

2. The nano-austenitic skeleton toughened high-ductility steel according to claim 1, characterized in that: The sub-rapid solidification cooling rate is 10. 2 ~10 3 k / s.

3. The nano-austenitic skeleton toughened high-ductility steel according to claim 1, characterized in that: The hot rolling temperature range is 800~950℃, and the reduction rate is 20~60%.

4. The nano-austenitic skeleton toughened high-ductility steel according to claim 1, characterized in that: The resulting strip steel has the following microstructure characteristics: nano-austenite is uniformly dispersed at the refined lath martensite grain boundaries and / or twin boundaries, forming a strong austenite framework structure that runs through the entire material; submicron-sized thin films and / or bulk austenite are uniformly distributed between the martensite laths.

5. The nano-austenitic skeleton toughened high-ductility steel according to claim 1, characterized in that: The yield strength of the obtained strip is 850~950MPa, the tensile strength is >1000MPa, the elongation after fracture is ≥30%, and the strength-ductility product is 30~40GPa.

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