High ductility low cost steel with tensile strength > 2600 mpa and method of making

Abstract

The application relates to the field of ultra-high-strength steel preparation and provides a high-plasticity low-cost steel with a tensile strength greater than 2600 MPa and a preparation method, the preparation method comprising the following steps: S1, alloy raw materials with a set ratio are smelted to be cast into a casting blank or a steel ingot; S2, the casting blank or the steel ingot is heated to a set temperature, is kept warm, is forged into a round or square cross-section forging blank in a rotating state through multiple passes, and is cooled to room temperature; at least one return-to-furnace keeping-warm treatment is carried out between each pass; and S3, the forging blank is subjected to low-temperature tempering treatment. The process of hot forging and direct tempering provided by the application solves the problems that traditional ultra-high-strength low-alloy steel is difficult to be industrialized and the production process is long; meanwhile, the application solves the problem that the strength and plasticity of ultra-high-strength steel are difficult to be considered together; the total elongation is more than 10% while the tensile strength is greater than 2600 MPa; and the application can be used in special engineering fields with extremely high requirements on strength and plasticity.

Description

Technical Field

[0001] This invention relates to the field of ultra-high strength steel preparation technology, and in particular to a high-ductility, low-cost steel with a tensile strength >2600MPa and its preparation method. Background Technology

[0002] Steel, with its high strength and excellent ductility, dominates the field of metallic structural materials. Ultra-high-strength steel, with a strength greater than 2.0 GPa, is of paramount importance in breakthrough applications and is considered a crucial material for groundbreaking applications in civil infrastructure, machinery, transportation, aerospace, and marine engineering. With increasing pressure on resources, energy, and the environment, the development of ultra-high-strength steel with superior ductility and toughness is receiving increasing attention. This is because, while meeting the same load-bearing capacity, ultra-high-strength steel can minimize material usage, thus achieving lightweighting requirements. This not only contributes to energy conservation and reduced greenhouse gas emissions, promoting the sustainable development of the steel industry, but also effectively addresses the demand for high-performance steel materials in a series of key national defense projects. In the automotive industry, ultra-high-strength steel has become an essential material for many automakers, used in the production of chassis, frames, roofs, and bodies, improving vehicle safety and fuel efficiency while reducing weight and carbon emissions. In the aerospace field, ultra-high-strength steel can be used to manufacture aircraft engine components, aerospace structural parts, and satellite components. In the energy sector, ultra-high strength steel can be used to manufacture turbine components, nuclear reactor parts, and oil and gas pipelines. The material can withstand high pressure, high temperature, and highly corrosive environments, while also possessing good toughness and weldability. In short, ultra-high strength steel has a wide range of applications and will have even broader prospects in the future. However, its low plasticity, high manufacturing cost, and difficulty in industrial application significantly limit its use.

[0003] Chinese patent CN 103898299A discloses a low-cost method for preparing 2400MPa grade nano-bainitic steel. The steel's alloy composition, by mass percentage, is: C: 0.5–1.0, Si: 2.0–3.0, Mn: 0.3–0.5, Al: 0.5–1.0, with the balance being Fe. The preparation method involves forging followed by salt bath quenching and holding. Although this steel uses inexpensive elements such as Al and Si as alloying elements, the addition of Al can cause clogging of the sprue during the casting process. Furthermore, salt bath quenching can cause environmental pollution. In addition, although the optimal composition and processing strength of this steel can reach a maximum of 2399MPa, the plasticity is significantly reduced at this point, with an elongation of only 3.1%.

[0004] Publication number CN110055392A discloses a high-toughness bridge cable steel with a tensile strength >2500MPa and its preparation method. This steel is produced by hot-drawing 14mm wire rod to 6.9mm, achieving a strength of 2500MPa and a torsion resistance of over 20 cycles. The microstructure is carbon-free bainite. Although this steel exhibits excellent strength and toughness, the drawing method is only suitable for preparing wire samples with small cross-sectional dimensions.

[0005] Chinese patent CN 113604753A discloses a 2700MPa-grade high-ductility, high-corrosion-resistant martensitic aging stainless steel and its preparation method. The alloy composition of this ultra-high-strength steel, by mass percentage, is: Cr: 11-17, Ni: 7-9, Co: 3-6, Mo: 5-7, Ti: 0.5-2. This steel is currently the ultra-high-strength steel with the best comprehensive mechanical properties reported in patent literature. Its strength is enhanced through the synergistic strengthening of Mo-rich R' phase, α-Cr phase, and Ni3(Ti,Mo) nanophase. The highest strength achieved with optimized composition and process can reach 2737MPa, with an elongation of 10.3%. However, the alloy content of this optimized composition is as high as 33.8%, resulting in extremely high costs. Furthermore, this steel is prepared by cold rolling, which requires extremely sophisticated equipment, making industrial production difficult.

[0006] The article “Li Junkui, Yang Zhinan, Ma Hua, et al. A medium-C martensite steel with 2.6 GPa Tensile strength and large ductility[J].Scripta Materialia,2023,228,115327” reports a high-strength (tensile strength: 2590 MPa) and high-ductility (total elongation 14.5%) martensite steel and its preparation method. The preparation of this alloy requires warm rolling at 400-600℃. The warm rolling deformation resistance is large, and the requirements for rolling equipment are extremely high, making it difficult to apply to industrial production.

[0007] The results of a search of existing technologies reveal that current ultra-high strength steels suffer from poor plasticity, excessively high alloy costs, and stringent manufacturing processes, making them unsuitable for large-scale application. In contrast, this invention features a simple alloy composition and low alloy content (<7%), and utilizes only a hot forging and tempering process, which is entirely feasible for industrial production, to achieve mechanical properties superior to those reported in the aforementioned literature. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of the prior art and provide a high-plasticity, low-cost steel with a tensile strength >2600MPa and its preparation method. The hot forging + tempering process eliminates the need for re-austenitization treatment after hot forging, as is required for steels such as 300M steel. This greatly simplifies the preparation process of ultra-high strength steel, saves preparation costs, and the hot forging + tempering process is completely industrially feasible and fully applicable to large-scale industrial production.

[0009] The present invention adopts the following technical solution:

[0010] On one hand, the present invention provides a method for preparing high-ductility, low-cost steel with a tensile strength >2600 MPa, comprising:

[0011] S1. Smelt the alloy raw materials with a set ratio and cast them into billets or steel ingots.

[0012] S2. Heat the billet or steel ingot to a set temperature, hold it at that temperature, and forge the billet or steel ingot in a rotating state through multiple passes into a forging billet with a circular or square cross section, and cool it to room temperature; wherein, at least one reheating and heat preservation treatment is performed between each pass of the multi-pass forging.

[0013] S3. The forging billet treated in step S2 is subjected to low-temperature tempering to obtain high-plasticity, low-cost steel with a tensile strength >2600MPa.

[0014] In addition to any of the possible implementations described above, another implementation is provided in which the mass percentage of the alloy raw material in step S1 is: C: 0.5-0.75%, Si: 0.2-2.5%, Mn: 0.2-2.0%, Cr: 0.2-2%, Ni: 0.3-3%, Mo+V+Nb: 0.4-3%, with the remainder being Fe and unavoidable impurity elements.

[0015] In addition to any of the possible implementations described above, a further implementation is provided in which, in step S2, the billet or steel ingot is heated to 950°C to 1250°C and held at that temperature for 1 to 3 hours.

[0016] In addition to any of the possible implementations described above, another implementation is provided in which, in step S2, a reheating treatment is performed between each pass of the multi-pass forging, and the reheating time is greater than 0.5 hours.

[0017] In addition to any of the possible implementations described above, another implementation is provided in which, in step S2, the forging ratio of the forging billet with a circular or square cross-section after being forged in a rotating state after being kept warm in the furnace is greater than 8, and the alloy structure after multiple forging passes is a layered structure.

[0018] In addition to any of the possible implementations described above, another implementation is provided in which, in step S2, the forging billet with a circular or square cross-section obtained by forging is cooled to room temperature by air cooling or by a cooling rate greater than that of air cooling, and the martensite volume fraction of the room temperature structure is not less than 90%.

[0019] In addition to any of the possible implementations described above, another implementation is provided in which the cooling method for cooling the forged billet with a circular or square cross-section to room temperature is air cooling, water mist cooling, water cooling, or liquid nitrogen cooling.

[0020] In addition to any of the possible implementations described above, another implementation is provided in which, in step S2, the final forging temperature of the multi-pass forging is higher than the Ar3 temperature and lower than the alloy dynamic recrystallization temperature.

[0021] In addition to any of the possible implementations described above, a further implementation is provided in which, in step S3, the tempering treatment is performed by: holding the forging billet at 80–300°C for 0.1–120 h to eliminate the internal stress generated during the forging and quenching processes, while simultaneously precipitating supersaturated carbon to form high-density fine carbides with Mo, V, and Nb, and then cooling it to room temperature to obtain the high-strength, high-plasticity, low-cost steel.

[0022] On the other hand, the present invention also provides a high-ductility, low-cost steel with a tensile strength >2600MPa, wherein the composition percentage is: CC: 0.5-0.75%, Si: 0.2-2.5%, Mn: 0.2-2.0%, Cr: 0.2-2%, Ni: 0.3-3%, Mo+V+Nb: 0.4-3%, and the remainder is Fe and unavoidable impurity elements;

[0023] The high-plasticity, low-cost steel is obtained by the above-described preparation method; the retained austenite accounts for 2 to 10% of the volume of the high-strength, high-plasticity, low-cost steel.

[0024] In addition to any of the possible implementations described above, another implementation is provided in which the alloying element content of the high-strength, high-plasticity, low-cost steel is less than 7%, and the total elongation is greater than 10%.

[0025] The principle of this invention is as follows:

[0026] This invention employs a low-alloy composition design, with a total alloy element content of <7wt%, resulting in low cost and economy. It utilizes a micro-alloying composition design, leveraging the precipitation of fine VC particles during hot deformation to strengthen the martensitic matrix while reducing the carbon content and inhibiting the formation of twinned martensite, which is detrimental to toughness. During forging, this invention employs forging above the austenitizing temperature and below the dynamic recrystallization temperature, which reduces forging deformation resistance, inhibits austenite recrystallization, and avoids the formation of coarse recrystallized primary austenite, thus preventing damage to toughness. This invention, through deformation below the recrystallization temperature, achieves a martensitic topology with multiple martensitic lath orientations, a structure that has proven to have excellent plasticizing and toughening effects in medium-manganese steel. This invention achieves a martensitic structure of over 90% through direct quenching after forging, ensuring ultra-high strength while retaining a certain amount of retained austenite. This retained austenite is distributed in a thin film form between layered martensite. This thin film-like retained austenite is more stable than blocky martensite and gradually transforms into martensite during deformation, providing good work hardening and thus excellent plasticity. Due to the excellent plasticity foundation provided by the retained austenite and layered martensite structure of the ultra-high strength steel of this invention, excellent plasticity can be obtained while maintaining high dislocation density and high solid solution strengthening effect. Therefore, to retain high dislocation density and high solid solution strengthening effect, and based on the minimum carbon atom precipitation temperature of 80℃, this invention broadens the tempering temperature range from 150–300℃ for general martensitic steel to 80–300℃ for the ultra-high strength steel of this invention. This broadens the lower limit of the tempering temperature for conventional tempering treatment of martensitic steel, thus successfully preparing ultra-high strength steel with a strength exceeding 2800 MPa while maintaining a total elongation exceeding 10%.

[0027] The beneficial effects of this invention are as follows:

[0028] (1) The present invention has a simple composition and low alloy content, resulting in extremely low cost compared to high-alloy high-strength steels such as martensitic aging steel. Compared to other low-cost alloy high-strength steels, the present invention exhibits significant advantages in comprehensive mechanical properties. The strength of the steel prepared by the preferred composition and process of the present invention can reach up to 2814 MPa, while maintaining excellent plasticity of 11.6%, making it the only ultra-high-strength steel reported in the literature to achieve a strength of 2800 MPa while maintaining an elongation of over 10%.

[0029] (2) The process of this invention is simple. Compared with room temperature and medium temperature large plastic deformations such as cold rolling and warm rolling, which are difficult to achieve industrial-scale production, this invention only adopts a simple process of hot forging + low temperature tempering, which is completely feasible in industry, and obtains comprehensive mechanical properties that are superior to those of ultra-high strength steels with large plastic deformations such as cold rolling and warm rolling. In addition, by directly tempering after hot forging, this invention omits the re-austenitizing quenching process of general low alloy high strength steel, which greatly simplifies the preparation process of traditional low alloy high strength steel.

[0030] (3) The steel of the present invention has excellent hardenability and flexible cooling process selection. It can be quenched into martensite by air cooling, wind cooling, water mist cooling, water cooling and liquid nitrogen cooling, etc., and the volume fraction of martensite is >90%, thereby ensuring that the ultra-high strength steel of the present invention has a strength of more than 2600MPa.

[0031] (4) Compared with conventional martensitic steel, which needs to be tempered at above 150°C to obtain a certain plasticity, the ultra-high strength steel of the present invention provides a good plasticity basis due to the martensitic layered topology and thin film-like retained austenite. Therefore, the present invention can be tempered at a lower temperature, thereby obtaining excellent performance with a strength greater than 2600MPa and a total elongation greater than 10% while retaining a higher dislocation density. Attached Figure Description

[0032] Figure 1 The diagram shown is a timing diagram of the preparation process in the embodiment.

[0033] Figure 2 The diagram shown is a flowchart illustrating a method for preparing a high-ductility, low-cost steel with a tensile strength > 2600 MPa according to an embodiment of the present invention.

[0034] Figure 3 The figure shows the engineering stress-strain curve of Example 2.

[0035] Figure 4 The X-ray diffraction pattern of Example 2 is shown.

[0036] Figure 5 The diagram shown is a distribution map of the thin-film austenite residual austenite in Example 2.

[0037] Figure 6 The diagram shown is a layered tissue diagram of Example 2.

[0038] Figure 7 This is a transmission electron microscope image of Example 2.

[0039] Figure 8 The diagram shown is a large isometric structure diagram of Comparative Example 2. Detailed Implementation

[0040] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be noted that the technical features or combinations of technical features described in the following embodiments should not be considered in isolation, but can be combined with each other to achieve better technical effects.

[0041] like Figure 1 As shown in the figure, an embodiment of the present invention provides a method for preparing a high-ductility, low-cost steel with a tensile strength > 2600 MPa, comprising:

[0042] S1. Smelt the alloy raw materials with a set ratio and cast them into billets or steel ingots.

[0043] S2. Heat the billet or steel ingot to a set temperature, hold it at that temperature, and forge the billet or steel ingot in a rotating state through multiple passes into a forging billet with a circular or square cross section, and cool it to room temperature; wherein, at least one reheating and heat preservation treatment is performed between each pass of the multi-pass forging.

[0044] S3. The forging billet treated in step S2 is subjected to low-temperature tempering to obtain high-plasticity, low-cost steel with a tensile strength >2600MPa.

[0045] Specific process sequence as follows Figure 2 As shown.

[0046] In one specific embodiment, in step S1, the mass percentage of the alloy raw material is: C: 0.5-0.75%, Si: 0.2-2.5%, Mn: 0.2-2.0%, Cr: 0.2-2%, Ni: 0.3-3%, Mo+V+Nb: 0.4-3%, with the remainder being Fe and unavoidable impurity elements.

[0047] In one specific embodiment, in step S2, the billet or steel ingot is heated to 950°C to 1250°C and held at that temperature for 1 to 3 hours.

[0048] In one specific embodiment, in step S2, a reheating treatment is performed between each pass of the multi-pass forging, and the reheating time is greater than 0.5 hours.

[0049] In one specific embodiment, in step S2, the forging ratio of the round or square cross-section forging blank after being forged in a rotating state after being kept warm in the furnace is greater than 8, and the alloy structure after multiple forging passes is a layered structure.

[0050] In one specific embodiment, in step S2, the forging billet with a circular or square cross-section obtained by forging is cooled to room temperature by air cooling or by a cooling rate greater than that of air cooling, and the martensite volume fraction of the room temperature structure is not less than 90%.

[0051] In one specific embodiment, the forging billet with a circular or square cross-section obtained by forging is cooled to room temperature by air cooling, water mist cooling, water cooling, or liquid nitrogen cooling.

[0052] In one specific embodiment, in step S2, the final forging temperature of the multi-pass forging is higher than the Ar3 temperature and lower than the alloy dynamic recrystallization temperature.

[0053] In one specific embodiment, in step S3, the tempering treatment is as follows: the forging billet is held at 80-300℃ for 0.1-120h to eliminate the internal stress generated during the forging and quenching processes, while supersaturated carbon precipitates to form high-density fine carbides with Mo, V and Nb, and then cooled to room temperature to obtain the high-strength, high-plasticity, low-cost steel.

[0054] This invention discloses a high-ductility, low-cost steel with a tensile strength >2600MPa, comprising the following composition percentages: C: 0.5-0.75%, Si: 0.2-2.5%, Mn: 0.2-2.0%, Cr: 0.2-2%, Ni: 0.3-3%, Mo+V+Nb: 0.4-3%, with the remainder being Fe and unavoidable impurity elements;

[0055] The high-plasticity, low-cost steel is obtained by the above preparation method; the retained austenite accounts for 2 to 10% of the volume of the high-strength, high-plasticity, low-cost steel.

[0056] The alloying element content of the high-strength, high-plasticity, low-cost steel is less than 7%, and the total elongation is greater than 10%.

[0057] The present invention will be further described below with reference to specific embodiments.

[0058] The chemical composition of each embodiment and comparative example is shown in Table 1, and the preparation process and mechanical properties of each embodiment and comparative example are shown in Table 2.

[0059] Table 1. Chemical composition (wt.%) of the embodiments and comparative examples of the present invention.

[0060] serial number C Mn Si Cr Ni Mo V Nb Example 1 0.48 0.4 0.8 0.8 0.8 1.2 0.7 0.1 Example 2 0.59 0.8 1.4 0.8 1.8 0.5 0.3 0 Example 3 0.59 0.8 1.4 0.8 1.8 0.5 0.3 0 Example 4 0.59 0.8 1.4 0.8 1.8 0.5 0.3 0 Example 5 0.59 0.8 1.4 0.8 1.8 0.5 0.3 0 Example 6 0.59 0.8 1.4 0.8 1.8 0.5 0.3 0 Example 7 0.65 2 0.2 1.8 0.3 0.3 0.1 0.5 Example 8 0.68 0.4 2.4 0.2 3 0.3 0.2 0 Comparative Example 1 0.59 0.8 1.4 0.8 1.8 0.5 0.3 0 Comparative Example 2 0.59 0.8 1.4 0.8 1.8 0.5 0.3 0

[0061] Table 2. Preparation process and mechanical properties of each embodiment and comparative example of the present invention.

[0062]

[0063]

[0064] As can be clearly seen from the mechanical properties in Table 2, the strength of all embodiments of the present invention is higher than 2600 MPa, and the elongation is higher than 10%. The optimal composition and process in the embodiments can achieve a strength of 2814 MPa while obtaining a high elongation of 11.6%. Figure 3 As shown. The excellent comprehensive mechanical properties of Example 2 are related to a certain amount of thin-film retained austenite and layered martensite topology, such as... Figure 4 As shown in Figures 5 and 6, the presence of a high density of fine carbides within the martensitic matrix provides excellent precipitation strengthening, such as... Figure 7As shown. In Comparative Example 1, the tempering temperature was too low, preventing carbon atoms from precipitating from the supersaturated martensite, thus failing to release quenching stress and causing brittle fracture during tensile testing. In Comparative Example 2, because the final forging temperature was above the dynamic recrystallization temperature, dynamic recrystallization occurred with larger grain sizes. These coarse recrystallized grains promoted the formation of brittle martensitic twins. Although a strength of over 2600 MPa could still be achieved, the plasticity loss was severe, below 10%. Figure 8 As shown in Table 2.

[0065] The hot forging and direct tempering process provided by this invention solves the problems of traditional ultra-high strength low alloy steel being difficult to industrialize (traditional ultra-high strength low alloy steel requires cold rolling, warm rolling, and other processes) and having a long production process (traditional ultra-high strength low alloy steel requires re-austenitizing quenching treatment between hot rolling / hot forging and tempering). Simultaneously, this invention solves the problem of balancing strength and plasticity in ultra-high strength steel, achieving a tensile strength greater than 2600 MPa while obtaining a total elongation exceeding 10%, making it suitable for special engineering fields with extremely high requirements for both strength and plasticity.

[0066] While several embodiments of the present invention have been provided herein, those skilled in the art should understand that modifications can be made to these embodiments without departing from the spirit of the invention. The above embodiments are merely exemplary and should not be construed as limiting the scope of the invention.

Claims

1. A method for preparing high-ductility, low-cost steel with a tensile strength > 2600 MPa, characterized in that, The preparation method includes: S1. The alloy raw materials with a set ratio are smelted and cast into billets or steel ingots; the mass percentage of the alloy raw materials is: C: 0.5~0.75%, Si: 0.2~2.5%, Mn: 0.2~2.0%, Cr: 0.2~2%, Ni: 0.3~3%, Mo+V+Nb: 0.4~3%, and the remainder is Fe and unavoidable impurity elements; S2. The billet or ingot is heated to a set temperature and held at that temperature. The billet or ingot is then forged in multiple passes while rotating to form a circular or square cross-section forging billet, which is then cooled to room temperature. At least one reheating and holding treatment is performed between each pass of the multi-pass forging process. The circular or square cross-section forging billet obtained by forging is cooled to room temperature by air cooling or a cooling rate greater than air cooling. The final forging temperature of the multi-pass forging is higher than the Ar3 temperature and lower than the alloy dynamic recrystallization temperature. S3. The forging billet treated in step S2 is subjected to low-temperature tempering to obtain high-plasticity, low-cost steel with a tensile strength >2600MPa. The low-temperature tempering is performed by holding the forging billet at 80~300℃ for 0.1~120h to eliminate the internal stress generated during the forging and quenching processes, while supersaturated carbon precipitates to form high-density fine carbides with Mo, V and Nb, and then cooling to room temperature to obtain the high-plasticity, low-cost steel.

2. The method for preparing high-ductility, low-cost steel with a tensile strength > 2600 MPa as described in claim 1, characterized in that, In step S2, the billet or steel ingot is heated to 950℃~1250℃ and held for 1~3 hours.

3. The method for preparing high-ductility, low-cost steel with a tensile strength > 2600 MPa as described in claim 1, characterized in that, In step S2, a reheating treatment is performed between each pass of the multi-pass forging process, and the reheating time is greater than 0.5 hours.

4. The method for preparing high-ductility, low-cost steel with a tensile strength > 2600 MPa as described in claim 1, characterized in that, In step S2, the forging ratio of the round or square cross-section forging billet after being forged in a rotating state after being kept warm in the furnace is greater than 8, and the alloy structure after multiple forging passes is a layered structure.

5. A high-ductility, low-cost steel with a tensile strength > 2600 MPa, characterized in that, The high-plasticity, low-cost steel is obtained by the preparation method described in any one of claims 1-4; the composition percentage of the high-plasticity, low-cost steel is: C: 0.5~0.75%, Si: 0.2~2.5%, Mn: 0.2~2.0%, Cr: 0.2~2%, Ni: 0.3~3%, Mo+V+Nb: 0.4~3%, with the remainder being Fe and unavoidable impurity elements; Retained austenite accounts for 2 to 10% of the volume of the high-ductility, low-cost steel.

6. The high-ductility, low-cost steel with a tensile strength > 2600 MPa as described in claim 5, characterized in that, The alloying element content of the high-plasticity, low-cost steel is less than 7%, and the total elongation is greater than 10%.

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