Preparation method of a twin martensite and nano austenite superplastic heterogeneous steel

Abstract

The present invention belongs to the technical field of metallurgical materials and relates to a preparation method for a superplastic heterogeneous steel of twinned martensite and nano austenite. The method of the present invention is as follows: molten steel is cast by sub-rapid solidification technology to obtain a casting sample; the casting sample is heated to the full austenitizing temperature range for short-time isothermal annealing, and then air-cooled to 800-900 °C for hot rolling, with a reduction ratio of 25-70%; the hot-rolled steel strip is air-cooled to a temperature range 50-100 °C below the Ms temperature for short-time isothermal tempering, and then immediately water-quenched to obtain the finished steel. The yield strength of the obtained product is 700-900 MPa, the tensile strength > 1000 MPa, the elongation after fracture > 33%, and the product of strength and plasticity is 34-40 GPa%. The present invention significantly expands the dimension of advanced high-strength steel material design, provides new research ideas and directions for the development of new advanced high-strength carbon steels with low cost and high thermal stability, and has broad application prospects.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgical materials, and in particular relates to a method for preparing twin martensite and nano-austenite superplastic heterogeneous steel. Background Art

[0002] Lightweight advanced high-strength steels with high ductility play an irreplaceable role in reducing carbon emissions, improving energy utilization and infrastructure safety. Generally, the realization of excellent comprehensive mechanical properties of high-strength steel requires the addition of a large amount of carbon (>0.4wt.%) and expensive alloy doping elements such as cobalt, nickel, and chromium. However, the use of high-strength steels with high carbon and high alloy content not only greatly increases the production cost, but also deteriorates its welding performance, which is very unfavorable for the promotion and industrialization of high-strength steel. Research in the past few decades has shown that introducing high-density lattice defects (including grain boundaries, phase boundaries, etc.) in the microstructure is one of the most effective ways to produce low-cost high-strength steel. The so-called grain boundary engineering is to achieve strengthening effects by utilizing planar discontinuities in metal crystals to adjust the mechanical properties of polycrystalline materials. The core of grain boundary engineering strengthening is to use the discontinuity of the crystal structure to organize martensite growth, and to limit its size and distribution by adjusting the number or arrangement of grain boundaries and phase boundaries to achieve strengthening effects. The fine grain strengthening and dislocation strengthening mechanisms that have been widely used to adjust the mechanical properties of advanced engineering materials belong to grain boundary engineering. However, due to the low thermal stability of crystal plane interfaces, grain coarsening easily occurs when the material is subjected to thermal loading, which results in some limitations on the fineness and type of microstructures that can be achieved by GBE.

[0003] As a new type of planar defect, chemical boundary engineering, which has been explored and developed in recent years, has been proven to be effective in breaking through these limitations, allowing us to create metal materials with ultra-fine graded non-uniform microstructures to enhance the mechanical properties and thermal stability of the materials. Chemical boundaries represent very sharp chemical discontinuities within the same lattice region. Chemical grain boundary engineering is achieved by organizing martensite growth by utilizing the local reduction of driving force caused by chemical discontinuities, limiting its size and distribution, and constructing a new microstructure that can act on the local phase change response of the material to achieve strengthening. The core of the strengthening effect of chemical grain boundary engineering is to obtain high-density and high-sharp chemical boundaries. The realization of high-density and high-sharp chemical boundaries usually requires a significant diffusion mismatch between fast-diffusing elements and slow-diffusing elements, which is generally obtained by rapid heating, and requires a large content difference between fast-diffusing elements and slow-diffusing elements. Therefore, the current successful application cases of chemical boundary engineering are mainly steels with high alloy doping elements such as medium manganese steel and high manganese steel. However, so far, there have been no reports on controlling the content difference between carbon and manganese elements to less than 2% and obtaining superplastic heterogeneous steels. Summary of the invention

[0004] In order to expand the dimension of material design, the present invention combines the characteristics of sub-rapid solidification with hot rolling at critical region temperature, extending the application of chemical grain boundary engineering to the field of carbon steel, maximizing the significant diffusion mismatch between carbon and manganese elements (the content difference between fast-diffusion elements and slow-diffusion elements < 2%), forming chemical boundaries with high density and high sharpness, making it a powerful barrier to restrict subsequent phase transformation within ultrafine (sub-micron) domains. This invention can produce a novel layered non-uniform microstructure composed of nano-film austenite and nano-scale lath martensite, and the sub-structure of the lath martensite is high-density nano-twins and high-density dislocations, enabling the steel to achieve an ultimate tensile strength exceeding 1.0 GPa and having ultra-high ductility (>30%) under the conditions of low carbon and low alloying element addition, which has important guiding significance for the development of new advanced high-strength carbon steels with low cost and high thermal stability.

[0005] The purpose of the present invention is to provide a design concept and method for a low-carbon, low-alloy lightweight advanced high-strength steel with high ductility. To achieve the above goal, the technical solution provided by the present invention is as follows:

[0006] A preparation method of a twin martensite and nano-austenite superplastic heterogeneous steel according to the present invention includes first casting a steel sample obtained by smelting through sub-rapid solidification technology; heating the steel sample to the full austenitization temperature range for short-time annealing (900 - 950 °C, 300 - 600 s), and then air-cooling to 800 - 900 °C for hot rolling, with a reduction ratio of 25 - 70%, preferably 25 - 60%, and further preferably 30 - 50%; air-cooling the hot-rolled steel strip to a temperature range 50 - 100 °C below the martensite start transformation temperature Ms, and holding at this temperature for 300 - 900 s, and then immediately water-quenching to obtain the finished steel. The steel water is composed of the following components by mass percentage:

[0007] C: 0.17 - 0.22%, Si: 1.5 - 2.0%, Mn: 1.6 - 2.0%, Cr ≤ 0.05%, P: ≤ 0.007%, S ≤ 0.002%, Al ≤ 0.003%, Ti ≤ 0.003%, and the rest is Fe and its inevitable impurities.

[0008] When applied industrially, steel water is obtained through smelting. Of course, the steel water can also be smelted by an electric furnace, a converter, or other methods, and then refined; the refining technology used can be selected from at least one of vacuum degassing refining and ladle refining. Of course, other refining technologies can also be used in the present invention.

[0009] As a preferred embodiment, for the preparation method of the twin martensite and nano-austenite superplastic heterogeneous steel of the present invention, the chemical composition of the molten steel in mass percentage is as follows: C: 0.18%, Si: 1.90%, Mn: 1.82%, Cr: 0.036%, Al: 0.0026%, Ti: 0.0022%, P: ≤0.007%, S ≤0.002%, and the rest is Fe and inevitable impurities.

[0010] As a preferred embodiment, for the preparation method of the twin martensite and nano-austenite superplastic heterogeneous steel of the present invention, the superheat of the molten steel is 50°C.

[0011] As a preferred embodiment, for the preparation method of the twin martensite and nano-austenite superplastic heterogeneous steel of the present invention, the cooling rate of the sub-rapid solidification technology is 900 - 2000 k / s.

[0012] As a preferred embodiment, for the preparation method of the twin martensite and nano-austenite superplastic heterogeneous steel of the present invention, the full austenitization temperature range is 30 - 80°C higher than the Ac3 temperature, and the isothermal annealing time is 300 - 600 s. As a further preference, the full austenitization temperature is 950°C and the annealing time is 300 s.

[0013] As a preferred embodiment, for the preparation method of the twin martensite and nano-austenite superplastic heterogeneous steel of the present invention, the hot rolling temperature range is 800 - 900°C. Further preferably, it is 850 - 900°C.

[0014] As a preferred embodiment, for the preparation method of the twin martensite and nano-austenite superplastic heterogeneous steel of the present invention, the reduction ratio of hot rolling is 30 - 50%.

[0015] As a preferred embodiment, for the preparation method of the twin martensite and nano-austenite superplastic heterogeneous steel of the present invention, the hot-rolled steel strip is air-cooled to a temperature range 80 - 90°C below the martensite start transformation temperature Ms, and is held at this temperature for 300 s.

[0016] As a preferred embodiment, for the preparation method of the twin martensite and nano-austenite superplastic heterogeneous steel of the present invention, it is immediately water-quenched after tempering to obtain the finished steel strip.

[0017] As a preferred embodiment, for the preparation method of the twin martensite and nano-austenite superplastic heterogeneous steel of the present invention, the structure of the obtained thin strip steel is: nano-film austenite is uniformly and dispersedly distributed among ultra-fine martensite laths, and the substructure of the lath martensite is high-density nano-twins and dislocations.

[0018] The present invention relates to a preparation method of a twinned martensite and nano austenite superplastic heterogeneous steel. The yield strength of the obtained strip steel is 700 - 900 MPa, the tensile strength > 1000 MPa, the elongation after fracture > 30%, and the product of strength and plasticity > 30 GPa%.

[0019] After optimization, the yield strength of the obtained hot-rolled thin strip steel is 850 - 900 MPa, the tensile strength is 1050 - 1080 MPa, the elongation after fracture is 35 - 36.5%, and the product of strength and plasticity is 39 - 40 GPa%.

[0020] In the present invention, C is the most basic element for stabilizing austenite and is an essential strengthening element in high-strength steels. In the present invention, due to its high diffusion rate, it can also be evenly distributed under the condition of sub-rapid solidification. On the one hand, by utilizing its function of stabilizing austenite, during the tempering stage, C enriches from supersaturated martensite into austenite, enabling austenite to be stably retained to room temperature. On the other hand, by utilizing the characteristic of the high diffusion coefficient of the C element, the diffusion mismatch between fast-diffusing C and slow-diffusing Mn is used to construct a high-density chemical boundary. The present invention can adjust the contents of fast-diffusing elements and slow-diffusing elements and the cooling rate of sub-rapid solidification to control the density of the chemical boundary. However, the C content needs to be within a suitable range to maximize its function of stabilizing retained austenite. If the C content is too high, it will promote the precipitation of carbides, thereby reducing the proportion of C in the retained austenite in the structure, and ultimately resulting in insufficient stability of the retained austenite. In the present invention, the C content is 0.17 - 0.22%, and the preferred content is 0.18 - 0.2%.

[0021] In the present invention, Mn is a strong austenite stabilizing element. By improving the stability of austenite, a part of austenite is retained to room temperature. At the same time, Mn also has the function of improving the hardenability of the steel and, in combination with hot deformation at high temperature, plays a role in refining grains. In the present invention, by utilizing the characteristics of Mn element for stabilizing austenite and its small diffusion coefficient and slow diffusion, microsegregation is extremely easy to form under the condition of sub-rapid solidification. The significant diffusion mismatch between fast-diffusing C and slow-diffusing Mn is used to construct a high-density chemical boundary (especially controlling the content difference between carbon element and manganese element to be less than 2% is more conducive to the construction of a high-density chemical boundary). The contents of fast-diffusing elements and slow-diffusing elements and the cooling rate of sub-rapid solidification can be adjusted to control the density of the chemical boundary, which in turn restricts the martensite phase transformation during quenching, resulting in the final formation of a fine lath martensite and nano-film austenite stratified heterogeneous structure. Therefore, in the preferred scheme of the present invention, the content of Mn is controlled at 1.6 - 2.0 wt%.

[0022] In the present invention, the solubility of Si element in carbides is very low, which can strongly inhibit the formation of carbide precipitates, reduce the loss of C, enrich C in austenite as much as possible, and increase the proportion of austenite. However, too high Si content will seriously deteriorate the surface quality and machining performance of steel. Therefore, the Si content in the present invention is 1.4 - 2.0%, and the preferred content is 1.7 - 1.9%.

[0023] The addition of the remaining elements is mainly adjusted according to the actual situation and the required performance as appropriate, which is well-known technology in the art or exists as impurity elements.

[0024] The present invention extends the application of chemical grain boundary engineering to carbon steel by combining the characteristics of sub-rapid solidification and hot rolling in the critical temperature region, achieving a significant diffusion mismatch between carbon and manganese elements to the greatest extent (the content difference between fast-diffusion elements and slow-diffusion elements < 2%), and greatly expanding the dimension of the design of advanced high-strength structural materials.

[0025] The coupling effect of the inheritance of the sub-rapid solidification characteristics and the austenitizing temperature, annealing time, hot rolling temperature, reduction ratio, tempering temperature and time in the present invention is the key to a method for preparing a twin martensite and nano-austenite superplastic heterogeneous steel in the present invention. In the whole process, too high austenitizing temperature and too long annealing time will coarsen the structure of the cast strip, destroy the superiority of the cast strip structure brought by the sub-rapid solidification characteristics, and cannot obtain a high-sharpness chemical boundary. Too high hot rolling temperature and too low hot rolling reduction ratio will result in coarse prior austenite grains, which is not conducive to the formation of a high-density chemical boundary. Too high tempering temperature will reduce the proportion of martensite in the structure and cannot obtain the desired strength, while too low tempering temperature will reduce the proportion of austenite and reduce plasticity. Too long or too short tempering time will affect the stability of austenite and is not conducive to the occurrence of the TRIP effect. The austenitizing temperature, annealing time, hot rolling temperature and reduction ratio affect the inheritance of the sub-rapid solidification characteristics, and thus affect the formation of a high-density and high-sharpness chemical boundary, which has a crucial impact on the distribution and morphology of lath martensite and austenite in the final structure. The tempering temperature and time respectively affect the proportion of martensite and austenite in the final structure and the stability of austenite, and thus affect the occurrence of the TRIP effect. Only a good match of the austenitizing temperature, annealing time, hot rolling temperature, reduction ratio, tempering temperature and time can maximize the advantages of the sub-rapid solidification characteristics and make up for its deficiencies.

[0026] The structure and properties of the product obtained by the present invention have the following advantages:

[0027] Ran Ding et al. studied the microstructure and mechanical properties of medium manganese steel in Fe-0.2C-8Mn-2Al (wt.%) by combining traditional continuous casting technology with the Q-ART process, and finally obtained the optimal mechanical property combinations of ultimate tensile strength of 950 MPa, elongation of 34.5% and ultimate tensile strength of 1135 MPa, 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 an Fe-0.2C-8Mn-2Al medium Mn steel, Acta Materialia, Volume 147, 2018, Pages 59-69). Z.Y. Liang et al. studied medium manganese steel with a composition of Fe-7Mn-0.14C-0.23Si (wt.%), and the finally obtained optimal mechanical properties were ultimate tensile strength of 1003 MPa and elongation of 24.4% (Z.Y. Liang, Z.H. Cao, J. Lu, M.X. Huang, C.C. Tasan, Influence of co-existing medium Mn and dual phase steel microstructures on ductility and Lüders band formation, Acta Materialia, Volume 221, 2021, 117418). B.B. He et al. designed a new type of high-carbon medium manganese steel combining TWIP effect and TRIP effect, with a composition of Fe-0.45C-10Mn-1Al (wt.%), and finally achieved excellent mechanical properties of ultimate tensile strength of 1027 MPa and elongation of 46% (B.B. He, H.W. Luo, M.X. Huang, Experimental investigation on a novel medium Mn steel combining transformation-induced plasticity and twinning-induced plasticity effects, International Journal of Plasticity, Volume 78, 2016, Pages 173-186).The present invention relates to a preparation method of a twinned martensite and nano-austenite superplastic heterogeneous steel. Through the ingenious coupling of the characteristics of sub-rapid solidification with hot rolling and heat treatment processes, a novel layered non-uniform microstructure is generated, which consists of nano-film austenite and nano-scale lath martensite. The substructure of the lath martensite is high-density nano-twins and high-density dislocations, enabling the steel to achieve an ultimate tensile strength exceeding 1.0 GPa under the condition of low carbon and low alloying element addition (such as the Mn content is only 1.6 - 2.0%), and having ultra-high ductility (>33%), with a strength-ductility product of 34 - 40 GPa%, and the mechanical properties are comparable to those of medium manganese steel.

[0028] The cost advantages of the present invention are as follows:

[0029] The present invention utilizes the characteristics of sub-rapid solidification combined with hot rolling in the critical temperature range, extending the application of chemical grain boundary engineering to the field of carbon steel (C < 0.2%). The significant diffusion mismatch between carbon and manganese elements (the content difference between fast-diffusion elements and slow-diffusion elements < 2%) is maximally achieved, forming high-density and high-sharpness chemical boundaries, which become a powerful barrier restricting subsequent phase transformation within ultra-fine (sub-micron) domains. It significantly expands the dimension of advanced high-strength steel material design, which has important guiding significance for the development of new advanced high-strength carbon steel with low cost and high thermal stability. Brief Description of the Drawings

[0030] Figure 1 is the optical micrograph (OM) of the finished steel strip prepared in Example 1 of the present invention;

[0031] Figure 2 is the optical micrograph (OM) of the finished steel strip prepared in Example 2 of the present invention;

[0032] Figure 3 is the superimposed image of the electron backscatter diffraction (EBSD) phase map and the austenite inverse pole figure of the sample surface of the finished steel strip prepared in Example 1 of the present invention;

[0033] Figure 4 is the superimposed image of the electron backscatter diffraction (EBSD) phase map and the austenite inverse pole figure of the sample cross-section of the finished steel strip prepared in Example 1 of the present invention;

[0034] Figure 5 is the superimposed image of the electron backscatter diffraction (EBSD) phase map and the austenite inverse pole figure of the sample surface of the finished steel strip prepared in Example 2 of the present invention;

[0035] Figure 6 is the superimposed image of the electron backscatter diffraction (EBSD) phase map and the austenite inverse pole figure of the sample cross-section of the finished steel strip prepared in Example 2 of the present invention;

[0036] Figure 7It is the electron backscatter diffraction (EBSD) phase diagram of the surface of the finished steel strip prepared in Example 1 of the present invention. The red color represents austenite, and the green color represents martensite and ferrite;

[0037] Figure 8 It is the electron backscatter diffraction (EBSD) phase diagram of the cross-section of the finished steel strip prepared in Example 1 of the present invention. The red color represents austenite, and the green color represents martensite and ferrite;

[0038] Figure 9 It is the electron backscatter diffraction (EBSD) phase diagram of the surface of the finished steel strip prepared in Example 2 of the present invention. The red color represents austenite, and the green color represents martensite and ferrite;

[0039] Figure 10 It is the electron backscatter diffraction (EBSD) phase diagram of the cross-section of the finished steel strip prepared in Example 1 of the present invention. The red color represents austenite, and the green color represents martensite and ferrite;

[0040] Figure 11 It is the bright-field image of the twin substructure of lath martensite under a transmission electron microscope (TEM) of the sample of the finished steel strip prepared in Example 1 of the present invention;

[0041] Figure 12 It is the dark-field image of the twin substructure of lath martensite under a transmission electron microscope (TEM) of the sample of the finished steel strip prepared in Example 1 of the present invention;

[0042] Figure 13 It is the bright-field image of the twin substructure of lath martensite under a transmission electron microscope (TEM) of the sample of the finished steel strip prepared in Example 2 of the present invention;

[0043] Figure 14 It is the dark-field image of the twin substructure of lath martensite under a transmission electron microscope (TEM) of the sample of the finished steel strip prepared in Example 2 of the present invention;

[0044] Figure 15 It is the stress-strain curve diagram of the finished steel strips (products) prepared in Examples 1 and 2 of the present invention. Detailed implementation manners

[0045] A preparation method of a twin martensite and nano-austenite superplastic heterogeneous steel of the present invention is specifically implemented as follows:

[0046] 1) Steel melting

[0047] The molten steel is smelted, and the mass percentages of its chemical components are as follows: C: 0.17 - 0.22%, Si: 1.5 - 2.0%, Mn: 1.6 - 2.0%, Cr ≤ 0.05%, P: ≤ 0.007%, S ≤ 0.002%, Al ≤ 0.003%, Ti ≤ 0.003%, and the rest is Fe and its inevitable impurities;

[0048] 2) Casting by the sub-rapid solidification technique

[0049] The molten steel obtained by smelting is cast by the sub-rapid solidification technique to obtain a cast strip sample;

[0050] 3) Isothermal annealing

[0051] The cast strip sample is heated to the full austenitization temperature range for short-time annealing (900 - 950 °C, 300 - 600 s);

[0052] 4) Hot rolling

[0053] Immediately air-cool to 800 - 900 °C for hot rolling, and the reduction ratio is 25 - 50%;

[0054] 5) Isothermal tempering

[0055] The hot-rolled steel strip is air-cooled to 80 - 90 °C below the martensite start transformation temperature Ms, and held at this temperature for 300 - 600 s.

[0056] 6) Quenching treatment

[0057] The steel strip after tempering treatment is immediately water-quenched to obtain the finished steel strip.

[0058] Next, in combination with the accompanying drawings in the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be described in detail. Embodiments 1 - 2 and Example 3 as well as Comparative Examples 1 - 2 of the present invention are all produced by a preparation method of a twinned martensite and nano-austenite superplastic heterogeneous steel. The difference between Comparative Example 3 and Examples 1 - 2 as well as Comparative Examples 1 - 2 lies in the post-hot rolling treatment process. Comparative Example 3 is directly water-quenched after hot rolling. Note that the described examples are only partial examples of the present invention. Therefore, all other examples obtained by those of ordinary skill in the art based on the examples in the present invention without creative labor fall within the protection scope of the present invention.

[0059] Example 1:

[0060] Example 1: The chemical composition of the steel grade in mass percentage is as follows: C: 0.18%, Si: 1.90%, Mn: 1.80%, Cr: 0.036%, Al: 0.0026%, Ti: 0.0022%, P: ≤0.007%, S ≤0.002%, and the rest is Fe and inevitable impurities. After converter, vacuum degassing refining and ladle refining, the molten steel enters the tundish (superheat temperature 80°C), and is cast into an as-cast thin strip with a thickness of 2.5 mm through the sub-rapid solidification technology. The sub-rapid solidification cooling rate is 1200 - 1500 k / s. The as-cast strip sample is heated to the full austenitization temperature of 950°C for a short annealing treatment of 300 s, and then air-cooled to 850 - 900°C for hot rolling. The reduction ratio is 30%, and the number of hot rolling passes is 3. The thickness of the hot-rolled thin strip is 1.75 mm. The hot-rolled steel strip is air-cooled to 300°C (76°C lower than the Ms temperature) for a short isothermal tempering treatment of 300 s, and then immediately water-quenched to obtain the final product.

[0061] Figure 1 The metallographic picture showing the final structure of Example 1 is presented. Figure 3 and Figure 4 are respectively the superimposed diagrams of the electron backscatter diffraction (EBSD) phase diagram and the austenite inverse pole figure of the surface and cross-section of the finished steel strip sample prepared in Example 1 of the present invention, where red is austenite, dark gray is lath martensite, and light gray is ferrite. Figure 7 and Figure 8 are respectively the phase diagrams of the electron backscatter diffraction (EBSD) of the surface and cross-section of the finished steel strip sample prepared in Example 1 of the present invention, where red is austenite, and green is martensite and ferrite. Figure 11 and Figure 12 are respectively the bright-field image and dark-field image of the twin substructure of lath martensite under a transmission electron microscope (TEM) of the sample of the finished steel strip prepared in Example 1 of the present invention. The structure of the finally obtained finished steel strip is a novel layered non-uniform microstructure, composed of nano-film austenite, nano-scale lath martensite and a small amount of ferrite, and the substructure of the lath martensite is high-density nano-twins and high-density dislocations. Among them, the average width of the film austenite is less than 200 nm, and the average width of the lath martensite is less than 300 nm. Figure 15 The green line in corresponds to the mechanical properties of Example 1, with a yield strength of 702 MPa, a tensile strength of 1025 MPa, an elongation after fracture of 33.2%, and a strength-ductility product of 34.03 GPa%.

[0062] Example 2:

[0063] The process route of Example 2 is the same as that of Example 1. The differences between them are as follows: the initial thickness of the cast strip, the hot rolling reduction rate, the number of hot rolling passes, and the thickness of the cast strip after rolling are different. The initial thickness of the cast strip in Example 2 is 2.7 mm, the hot rolling reduction rate is 50%, the number of hot rolling passes is 4, and the thickness of the cast strip after rolling is 1.48 mm.

[0064] Figure 2 The metallographic picture showing the final structure of Example 2 is presented. Figure 5 and Figure 6 are respectively the superimposed diagrams of the electron backscatter diffraction (EBSD) phase diagram and the austenite inverse pole figure of the surface and cross-section of the finished steel strip sample prepared in Example 2 of the present invention, where red represents austenite, dark gray represents lath martensite, and light gray represents ferrite. Figure 9 and Figure 10 are respectively the phase diagrams of the electron backscatter diffraction (EBSD) of the surface and cross-section of the finished steel strip sample prepared in Example 2 of the present invention, where red represents austenite, and green represents martensite and ferrite. Figure 13 and Figure 14 are respectively the bright field image and the dark field image of the twin substructure of lath martensite under a transmission electron microscope (TEM) of the sample of the finished steel strip prepared in Example 2 of the present invention. The structure of the finally obtained finished steel strip is a novel layered non-uniform microstructure, composed of nano-thin film austenite, nano-scale lath martensite, and a small amount of ferrite, and the substructure of the lath martensite is high-density nano-twins and high-density dislocations. Among them, the average width of the thin film austenite is less than 100 nm, and the average width of the lath martensite is less than 200 nm. Figure 15 The red line corresponds to the mechanical properties of Example 2, with a yield strength of 865 MPa, a tensile strength of 1076 MPa, an elongation after fracture of 36.4%, and a strength-ductility product of 39.2 GPa%.

[0065] Example 3

[0066] Example 3 adopts a similar process to Example 1 and Example 2. The differences are as follows: The differences between Example 3 and Example 1 and Example 2 lie in the hot rolling reduction rate and the number of hot rolling passes. The hot rolling reduction rate of Example 3 is 70%, the number of hot rolling passes is 5, the initial thickness of the cast strip is 2.7 mm, and the thickness after rolling is 0.81 mm.

[0067] Comparative Example 1

[0068] Comparative Example 1 adopts a similar process to Example 2. The differences are as follows: The differences between Comparative Example 1 and Example 2 lie in the austenitizing temperature and the hot rolling temperature. The austenitizing temperature of Comparative Example 1 is 850 °C, and the hot rolling temperature is 700 - 750 °C.

[0069] Comparative Example 2

[0070] Comparative Example 2 uses a process similar to that of Example 1 and Example 2. The differences are as follows: The differences between Comparative Example 2 and Example 1 and Example 2 lie in the hot rolling reduction rate and the number of hot rolling passes. The hot rolling reduction rate of Comparative Example 2 is 20%, the number of hot rolling passes is 2 passes, the initial thickness of the cast strip is 2.2 mm, and the thickness after rolling is 1.76 mm.

[0071] Comparative Example 3

[0072] Comparative Example 3 uses a different process from that of Example 1 and Example 2. The differences are as follows: The differences between Comparative Example 3 and Example 1 and Example 2 lie in the post - hot - rolling treatment process. The specific process is as follows: The cast strip sample is heated to the austenitizing temperature of 950 °C for a short - time annealing treatment of 300 s, and then air - cooled to 850 - 900 °C for hot rolling. The reduction rate is 50%, the number of hot rolling passes is 3 passes, the initial thickness of the cast strip is 3.0 mm, the thickness of the thin strip after rolling is 1.5 mm, and the hot - rolled steel strip is immediately water - quenched.

[0073] Through a large number of experimental explorations, it is obtained that: The reasonable austenitizing temperature is 20 °C below Ac1 to 80 °C above Ac1, the annealing time is ~300 s, the hot - rolling temperature is 800 - 900 °C, preferably 850 - 900 °C, the hot - rolling reduction rate is 25% - 70%, preferably 30 - 50%, the tempering temperature is 80 - 90 °C below Ms, and the tempering time is ~300 s. By reasonably adjusting the austenitizing temperature, hot - rolling temperature, and reduction rate, different combinations of mechanical properties can be achieved. It can either achieve high strength and ultra - high elongation (Example 1 and Example 2), or achieve high strength and good elongation (Example 3), meeting different usage scenarios of the material.

[0074]

[0075] In summary, the results of the examples and comparative examples show that for the preparation method of a twin - martensite and nano - austenite superplastic heterogeneous steel of the present invention, by utilizing the characteristics of sub - rapid solidification and hot rolling in the critical - region temperature, the application of chemical grain - boundary engineering is extended to the field of carbon steel (C < 0.2%). The significant diffusion mismatch between carbon and manganese elements (the content difference between fast - diffusion elements and slow - diffusion elements < 2%) is maximally achieved, forming a chemical boundary with high density and high sharpness, which becomes a powerful barrier restricting subsequent phase transformation within ultra - fine (sub - micron) domains. The present invention significantly expands the dimension of advanced high - strength steel material design, providing new research ideas and directions for the development of new advanced high - strength carbon steels with low cost and high thermal stability, and having broad application prospects.

Claims

1. A preparation method of a twin martensite and nano austenite superplastic heterogeneous steel, characterized in that It includes the following steps: casting the molten steel obtained by smelting into a casting sample through the sub-rapid solidification technology; heating the casting sample to the full austenitization temperature range for short-time annealing, and then air-cooling it to 800-900 °C for hot rolling, with a reduction ratio of 25-70%; air-cooling the hot-rolled steel strip to a temperature range 50-100 °C below the martensite start transformation temperature Ms, and performing short-time isothermal tempering treatment at this temperature, and then immediately water quenching to obtain the finished steel; the temperature of the short-time annealing is 900-950 °C, and the time is 300-600 s; The full austenitization temperature range is 900-950 °C; The molten steel is composed of the following components by mass percentage: C: 0.17-0.22%, Si: 1.5-2.0%, Mn: 1.6-2.0%, Cr ≤ 0.04%, P: ≤ 0.007%, S ≤ 0.002%, Al ≤ 0.003%, Ti ≤ 0.003%, and the rest is Fe and its inevitable impurities.

2. The preparation method of a twin martensite and nano-austenite superplastic heterogeneous steel according to claim 1, characterized in that: The superheat of the molten steel is 50-100 °C.

3. The preparation method of a twinned martensite and nano-austenite superplastic heterogeneous steel according to claim 1, characterized in that: The sub-rapid solidification cooling rate is 10 2 ~10 3 k / s.

4. The preparation method of a twinned martensite and nano-austenite superplastic heterogeneous steel according to claim 1, characterized in that: The hot rolling temperature range is 800-900 °C, and the reduction ratio is 25-60%.

5. The preparation method of a twin martensite and nano austenite superplastic heterogeneous steel according to claim 1, characterized in that: The structure of the obtained steel strip is: nano-film austenite is uniformly and dispersedly distributed among ultra-fine martensite laths, and the sub-structure of the lath martensite is high-density nano-twins and dislocations.

6. The preparation method of a twinned martensite and nano-austenite superplastic heterogeneous steel according to claim 1, characterized in that: The yield strength of the obtained steel strip is 700-900 MPa, the tensile strength > 1000 MPa, the elongation after fracture > 33%, and the strength-ductility product is 30-40 GPa%.

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