A multi-element microalloy high-toughness ultra-high-strength steel and a preparation method thereof

Abstract

The present application relates to a kind of multi-element microalloy high toughness ultra-high strength steel and its preparation method, belong to the technical field of ultra-high strength alloy steel.Solve the toughness problem of poor existing multi-element microalloy ultra-high strength steel.A kind of multi-element microalloy high toughness ultra-high strength steel, alloy composition weight percentage is as follows:C:0.36~0.38%, Mn:1.10~2.0%, Si:0.8~1.40%, Cr:1.10~1.50%, Ni:0.15~0.40%, Mo:0.2~0.5%, W:0.1~0.7%, Nb≤0.01%, V≤0.01%, B≤0.001%, the balance is Fe and inevitable impurities;Preparation process includes forging-annealing-normalizing-annealing-quenching-tempering.The multi-element microalloy high toughness ultra-high strength steel prepared has low cost, high hardenability, excellent performance.

Description

Technical Field

[0001] This invention relates to the field of ultra-high strength alloy steel technology, and in particular to a multi-element microalloyed high-toughness ultra-high strength steel and its preparation method. Background Technology

[0002] Low-alloy ultra-high-strength steel has broad market application prospects due to its relatively low alloy cost. Currently, the main steel grades on the market include 4340, D6AC, 300M, and 35CrMnSi. These products have been developed and used for over thirty years. However, due to constraints imposed by objective factors such as alloy smelting conditions, element costs, hot deformation, and heat treatment conditions at the time, as well as subjective factors such as an incomplete and in-depth understanding of physical metallurgy principles, the overall mechanical properties of this type of material are generally characterized by high strength and low toughness (tensile strength 1900 MPa, fracture toughness approximately 70 MPa·m). 1 / 2 (and low strength with high toughness (tensile strength 1700 MPa, fracture toughness approximately 100 MPa·m)) 1 / 2 Performance range. Compared with low-alloy ultra-high strength steel, although high-alloy secondary hardening steels such as A100 and M54 can meet the requirements of high strength and high toughness, their high cost and smelting expenses severely limit the application of high-toughness ultra-high strength steel in my country's aerospace and other fields.

[0003] Besides strength and toughness, low-alloy ultra-high-strength steel, due to its low alloy content, has a significant impact on the hardenability of different parts of the material. Hardenability refers to the ability of martensite phase formation to penetrate the entire cross-sectional thickness of the material during quenching, not just the surface layer. Hardenability is one of the important indicators for evaluating the quenching performance of a material, especially when a uniform martensitic phase transformation is required across the entire cross-section. Reasonably adjusting the hardenability to match the uniformity of the alloy's microstructure and properties also presents a considerable challenge.

[0004] Therefore, given the constraints of low alloy cost and low smelting cost, developing an alloy steel that can balance high strength and high toughness is of great significance. Summary of the Invention

[0005] Based on the above analysis, the present invention aims to provide a multi-element microalloyed high-toughness ultra-high-strength steel and its preparation method, so as to solve the problem of poor toughness of existing multi-element microalloyed ultra-high-strength steels.

[0006] On the one hand, the present invention provides a multi-element microalloyed high-toughness ultra-high-strength steel, the alloy composition of which, by weight percentage, is: C: 0.36-0.38%, Mn: 1.10-2.0%, Si: 0.8-1.40%, Cr: 1.10-1.50%, Ni: 0.15-0.40%, Mo: 0.2-0.5%, W: 0.1-0.7%, Nb≤0.01%, V≤0.01%, B≤0.001%, with the balance being Fe and unavoidable impurities.

[0007] Furthermore, the microstructure of the multi-element microalloyed high-toughness ultra-high-strength steel consists of martensite and thin-film austenite.

[0008] Furthermore, the volume fraction of the thin film austenite is 3% to 7%.

[0009] On the other hand, the present invention also provides a method for preparing multi-element microalloyed high-toughness ultra-high-strength steel, which includes the following steps:

[0010] Step S1: Forging. The steel billet that meets the alloy composition requirements is heated in a soaking furnace and then forged to obtain the first forged billet.

[0011] Step S2: First annealing, anneal the first forging billet and air cool it to obtain the second forging billet;

[0012] Step S3: Normalizing. The second forging billet is normalized and then air-cooled to room temperature to obtain the third forging billet.

[0013] Step S4: Second annealing. The third forging billet is heated for annealing and then air-cooled to room temperature to obtain the fourth forging billet.

[0014] Step S5: Quenching. The fourth forging billet is heated and held at a certain temperature for quenching treatment. Then it is cooled to room temperature by slow furnace cooling and oil cooling to obtain the fifth forging billet.

[0015] Step S6: Tempering. The fifth forging billet undergoes a first tempering and a second tempering. The second tempering temperature is not higher than the first tempering temperature, resulting in multi-element microalloyed high-toughness ultra-high-strength steel.

[0016] Furthermore, in step S1, the initial forging temperature is 1050℃~1150℃, and the final forging temperature is 920℃~950℃.

[0017] Furthermore, in steps S2 and S4, the annealing temperature is 600–650°C, and the annealing time is 2–10 hours.

[0018] Furthermore, in step S3, the normalizing temperature is 950–980°C, and the annealing time is 2–10 hours.

[0019] Furthermore, in step S5, the heat preservation temperature is 930℃~960℃, and the heat preservation time is 1~4h;

[0020] The furnace slow cooling and oil cooling process is as follows: the furnace is cooled to 830℃~860℃ and then oil cooled to room temperature.

[0021] Furthermore, in step S6, the first tempering temperature is 235℃~255℃, and the tempering time is 3~5h; the second tempering temperature is 225℃~235℃, and the tempering time is 3~5h.

[0022] Furthermore, in step S6, a cold treatment operation is also included between the first tempering and the second two-step tempering operation.

[0023] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0024] 1. This invention achieves a balance between high toughness and ultra-high strength in materials through the rational design of multi-element microalloying components and preparation processes. Precise control of the alloy composition and optimized grain boundary treatment enable the material to achieve excellent strength and toughness under low-alloy conditions, meeting the application requirements of high-strength materials in aerospace and defense fields.

[0025] 2. This invention achieves the preparation of multi-element microalloyed high-toughness ultra-high-strength steel through precise alloy composition design and precise control of process parameters. The microstructure of the prepared multi-element microalloyed high-toughness ultra-high-strength steel consists of martensite and thin-film austenite. This microstructure possesses the characteristics of small size and uniform distribution of thin-film austenite, effectively improving the strength and toughness of the material. Despite limitations in alloy cost, this invention enables high-strength steel to possess not only excellent strength but also outstanding toughness.

[0026] 3. This invention, by setting a high quenching temperature, provides sufficient heat energy to fully dissolve inclusions such as carbides into the matrix, preventing their precipitation at grain boundaries or within crystals, thus reducing the negative impact of grain boundary embrittlement on the material's toughness and strength. Through a two-stage quenching process of slow furnace cooling followed by oil cooling, the grain boundary segregation effect of elements such as B, Mo, and W is utilized to effectively purify grain boundaries, improve interfacial bonding, increase grain boundary strength, inhibit the nucleation of ferrite and bainite at grain boundaries, and enhance austenite stability. Simultaneously, it adjusts the overall temperature uniformity of the material, effectively avoiding the non-uniformity of microstructure and properties during the cooling process caused by size effects, and effectively improving the hardenability and the balance between strength and toughness.

[0027] 4. This invention avoids carbide formation and prevents grain boundary segregation of elements such as Cr and Mn by employing a generally lower tempering temperature strategy. The use of a two-stage low-temperature tempering process effectively reduces the retention of bulk austenite, preventing premature stress concentration during deformation and thus avoiding accelerated crack propagation. Compared to traditional microalloyed ultra-high-strength steel, the multi-element microalloyed steel prepared by this invention exhibits significantly improved fracture toughness.

[0028] 5. This invention introduces a cold treatment step, namely a cooling process performed between two tempering steps, to further control the residual blocky austenite. Through reasonable cold treatment process parameters, the phase transformation from blocky austenite to martensite can be achieved, effectively controlling the volume fraction of residual austenite and further improving the toughness of the material.

[0029] In summary, compared with the prior art, this invention improves the hardenability of the material and significantly enhances the toughness of microalloyed ultra-high strength steel by carefully designing the microstructure, rationally controlling the material composition, and optimizing the preparation process. This results in a significant improvement in its overall performance and provides a more superior and reliable material choice for applications in related fields.

[0030] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0031] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0032] Figure 1 This is a three-dimensional atomic probe image of the austenite grain boundaries in the untempered state of Embodiment 1 of the present invention;

[0033] Figure 2 This is a microstructure SEM image of Embodiment 1 of the present invention;

[0034] Figure 3 This is a TEM image of the microstructure in Example 4 of the present invention;

[0035] Figure 4 This is a TEM image of the microstructure of Comparative Example 2 of this invention;

[0036] Figure 5 This is a schematic diagram of the two-stage cooling and quenching-two-times tempering process of the present invention.

[0037] Figure label:

[0038] 1-Austenitic grain boundary; 2-Thin film austenite; 3-Carbide precipitation. Detailed Implementation

[0039] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0040] This invention discloses a multi-element microalloyed high-toughness ultra-high-strength steel. The alloy composition of this multi-element microalloyed high-toughness ultra-high-strength steel, by weight percentage, is: C: 0.36-0.38%, Mn: 1.10-2.0%, Si: 0.8-1.40%, Cr: 1.10-1.50%, Ni: 0.15-0.40%, Mo: 0.2-0.5%, W: 0.1-0.7%, Nb≤0.01%, V≤0.01%, B≤0.001%, with the balance being Fe and unavoidable impurities.

[0041] The reasons for limiting the composition of the billet in the multi-element microalloyed high-toughness ultra-high-strength steel and its preparation method in this invention will be explained. Hereinafter, only the weight percentage of the composition is expressed as %.

[0042] Carbon (C): Carbon is a key element affecting the strength of steel, but excessive carbon content will lead to excessive precipitation of carbides, which weakens the toughness of the material. In this invention, the C content is controlled at 0.36-0.38% to ensure that the steel has sufficient strength, while avoiding excessive precipitation of carbides and maintaining appropriate toughness.

[0043] Manganese (Mn): Manganese can effectively enhance the strength and stability of steel, and at the same time help strengthen grain boundaries. In this invention, the content of Mn is controlled at 1.10-2.0%.

[0044] Silicon (Si): A suitable silicon content helps improve the oxidation resistance and strength of steel, inhibits the precipitation of carbides during tempering, and also plays a certain role in strengthening the alloy; in this invention, the Si content is controlled at 0.80 to 1.40%.

[0045] Chromium (Cr): Chromium can provide good corrosion resistance and hardness, and also plays a positive role in grain boundary strengthening; the Cr content in this invention is controlled at 1.10-1.50%.

[0046] Nickel (Ni): Nickel can improve the toughness and corrosion resistance of steel, and also contributes to the strengthening and stability of alloys. In this invention, the Ni content is controlled at 0.15-0.40%.

[0047] Molybdenum (Mo): The addition of molybdenum helps to purify grain boundaries and improve grain boundary strength, thereby enhancing the toughness and stability of steel; in this invention, the content of Mo is controlled at 0.20-0.5%.

[0048] Tungsten (W): The addition of tungsten can further purify the grain boundaries, improve the grain boundary bonding force, and inhibit the nucleation of ferrite and bainite at the grain boundaries, thereby enhancing the stability of austenite; the content of W in this invention is controlled at 0.10 to 0.7%.

[0049] Niobium (Nb) and Vanadium (V): These two microalloying elements, Nb and V, play a pinning role at grain boundaries, which can effectively delay the growth of austenite grains and improve the toughness of the material. In this invention, the content of Nb and V is limited to a low level, both ≤0.01%.

[0050] Boron (B): The addition of boron helps to purify the austenite grain boundaries, improve the hardenability of the alloy, and increase the strength of the grain boundaries. However, the addition of excessive B will impair the toughness of the material. In this invention, the content of B is controlled at ≤0.001%.

[0051] It should be noted that alloying matrix elements such as Mn, Si, Cr, and Ni are crucial for providing strength and stability. Through reasonable configuration and control, uniform strengthening of the matrix can be achieved, thereby enhancing the overall performance of the material. Introducing Nb and V microalloying elements, utilizing their pinning effect at austenite grain boundaries, effectively delays austenite grain growth, maintaining a fine grain structure and improving the material's strength and toughness. Adding B, Mo, and W microalloying elements can purify austenite grain boundaries, improve grain boundary bonding and strength, enhance hardenability, and increase austenite stability. This invention fully utilizes the strengthening and toughening advantages of the aforementioned elements, rationally designing the alloy element ratios to allow them to interact during heat treatment, optimizing the grain boundary structure, and further improving the material's toughness and strength.

[0052] Preferably, a multi-element microalloyed high-toughness ultra-high-strength steel is provided, wherein the alloy composition of the multi-element microalloyed high-toughness ultra-high-strength steel by weight percentage is: C: 0.36-0.38%, Mn: 1.2-1.8%, Si: 0.9-1.40%, Cr: 1.12-1.50%, Ni: 0.20-0.40%, Mo: 0.25-0.5%, W: 0.15-0.5%, Nb≤0.01%, V≤0.008%, B≤0.0008%, with the balance being Fe and unavoidable impurities.

[0053] A specific embodiment of the present invention also discloses a method for preparing multi-element microalloyed high-toughness ultra-high-strength steel, which includes the following steps:

[0054] Step S1: Forging. The steel billet that meets the alloy composition requirements is heated in a soaking furnace and then forged to obtain the first forged billet.

[0055] Step S2: First annealing, anneal the first forging billet and air cool it to obtain the second forging billet;

[0056] Step S3: Normalizing. The second forging billet is normalized and then air-cooled to room temperature to obtain the third forging billet.

[0057] Step S4: Second annealing. The third forging billet is heated for annealing and then air-cooled to room temperature to obtain the fourth forging billet.

[0058] Step S5: Quenching. The fourth forging billet is heated and held at a certain temperature for quenching treatment. Then it is cooled to room temperature by slow furnace cooling and oil cooling to obtain the fifth forging billet.

[0059] Step S6: Tempering. The fifth forging billet undergoes a first tempering and a second tempering. The second tempering temperature is not higher than the first tempering temperature, resulting in multi-element microalloyed high-toughness ultra-high-strength steel.

[0060] Specifically, in step S1, the forging billet is obtained by vacuum induction combined with electroslag remelting or refining-casting-electroslag remelting; for example, the forging billet is in the form of a forging bar, and the diameter of the forging bar is within 400mm to ensure the hardenability requirements of the material.

[0061] The initial forging temperature is 1050℃~1150℃, and the final forging temperature is 920℃~950℃, with a forging ratio ≥5 during the forging process. Within this final forging temperature range, the segregation of elements such as Nb and V at grain boundaries can be prevented, inhibiting the recrystallization of austenite grains and affecting grain refinement. When the final forging temperature is below 920℃, the forged billet needs to be reheated in the furnace. Preferably, the initial forging temperature is 1050℃~1100℃, and the final forging temperature is 930℃~940℃.

[0062] Specifically, in step S2, the first forging billet is sent to an annealing furnace for the first annealing. The annealing temperature is 600-650℃, and the annealing time is 2-10 hours. After annealing, it is air-cooled to room temperature. During the annealing process, the material undergoes temperature rise and fall, which helps to eliminate residual stress generated during forging, improve the overall stability of the material, and reduce the risk of possible cracks and deformation.

[0063] Specifically, in step S3, the second forging billet is heated to 950℃~980℃ for normalizing treatment for 2~10 hours, and then air-cooled to room temperature to obtain the second forging billet.

[0064] Specifically, in step S4, after the forging billet completes the normalizing treatment and cools to room temperature, a second annealing treatment is performed within 4 hours to avoid the formation of microcracks caused by internal stress due to excessive cooling time. The second forging billet is heated to 600℃~650℃ for a second annealing treatment, with an annealing time of 2~10 hours, and then air-cooled to room temperature. After cooling, the microstructure of the forging billet is ferrite + pearlite + a small amount of bainite.

[0065] Specifically, in step S5, the third forging billet is heated and held at a temperature of 930℃~960℃ for 1~4 hours, followed by cooling. The quenching cooling process consists of two stages: slow cooling in the furnace and oil cooling. After quenching, the forging billet is slowly cooled in the furnace to 830℃~860℃, and then oil-cooled to room temperature. Preferably, after quenching, the forging billet is slowly cooled in the furnace to 845℃~855℃, and the slow cooling process does not exceed 5 hours. This operation is more conducive to the grain boundary segregation effect of B, Mo, and a small amount of W elements, while suppressing the formation of carbides and ferrite and bainite during quenching and cooling. By improving the quenching temperature difference and reducing alloy segregation, the hardenability of the material, especially large-sized forgings, is improved, thereby improving the performance stability of materials of different sizes.

[0066] Specifically, in step S6, the fourth forging billet undergoes a first tempering and a second tempering. The second tempering temperature is no higher than the first tempering temperature. The first tempering temperature is 235℃~255℃, and the tempering time is 3~5 hours; the second tempering temperature is 225℃~235℃, and the tempering time is 3~5 hours. The first tempering aims to achieve stress-relieving tempering, thereby improving the internal properties of the forging and forming carbon clusters in the material to pin dislocations, further enhancing the material's strength. After the first tempering, the internal stress of the material is released to a certain extent, and the static pressure on the retained austenite decreases, with some blocky retained austenite transforming into newly formed martensite. After the second tempering, the internal stress of the material is further improved, and the potential high-dislocation brittle newly formed martensite is softened, effectively avoiding stress concentration. At the same time, the retained austenite content of the forging billet is reduced to 3~7%, and the retained austenite is in the form of a thin film, which can effectively hinder crack propagation and improve fracture toughness.

[0067] Preferably, in step S6, a cold treatment operation is included between the first tempering and the second tempering; after the first tempering operation, the forging billet is air-cooled to room temperature and then cold-treated, followed by the second tempering operation. This operation ensures sufficient transformation of blocky retained austenite in larger bar stock (section diameter greater than 200 mm). For example, the cold treatment temperature is -100℃ to -50℃, and the cold treatment time is 1 to 2 hours. Figure 5 As shown, the process of two-stage cooling and quenching and two-stage tempering of the present invention is specifically demonstrated.

[0068] This invention achieves the preparation of multi-element microalloyed high-toughness and ultra-high-strength steel through precise alloy composition design and precise control of process parameters. By adding elements such as B, Mo, and W, this invention purifies and strengthens grain boundaries, effectively improving their bonding strength and stability. The presence of these elements successfully suppresses the precipitation of ferrite and bainite phases at grain boundaries, further enhancing the stability of the austenite phase. Although W and Mo readily form carbides during medium- and low-temperature heating and cooling, precise control of the austenitizing temperature eliminates the influence of carbides on the material, preventing potential grain growth and maintaining the material's strength and toughness. Simultaneously, the introduction of Nb and V effectively delays austenite grain growth, creating an effective pinning effect at grain boundaries, further improving the material's toughness. The preparation method of this invention employs annealing after normalizing and cooling to room temperature, resulting in more uniform carbide precipitation, reduced elemental segregation, and a fine, uniformly distributed austenite grain structure before quenching. A lower tempering temperature was used to avoid the formation of carbides and the interfacial segregation of elements such as Cr and Mn.

[0069] The multi-element microalloyed high-toughness ultra-high-strength steel prepared by the method of this invention has a microstructure consisting of martensite and thin-film austenite; wherein the volume fraction of thin-film austenite is 3%–7%. The steel obtained by this method exhibits a tensile strength >1950 MPa, with a maximum of 2010 MPa; a yield strength >1500 MPa, with a maximum of 1606 MPa; an elongation after fracture >10.5%, with a maximum of 12.5%; a reduction of area >40%, with a maximum of 48%; and a fracture toughness >90 MPa·m. 1 / 2 The maximum pressure can reach 104.5 MPa·m 1 / 2 This achievement demonstrates that, despite the cost constraints of low-alloy steel, the toughness of ultra-high-strength steel can be successfully improved, giving high-strength steel both good strength and toughness properties.

[0070] Example 1

[0071] The alloy composition of the raw steel billet in this embodiment, by weight percentage, is: C: 0.36%, Mn: 1.4%, Si: 1.3%, Cr: 1.15%, Ni: 0.28%, Mo: 0.35%, W: 0.25%, Nb: 0.005%, V: 0.008%, B: 0.0003%, with the balance being Fe and unavoidable impurities.

[0072] The preparation steps are as follows:

[0073] Step S1: Forging. The steel billet that meets the alloy composition requirements is heated in a soaking furnace and then forged to obtain the first forged billet.

[0074] The initial forging temperature is 1050℃ and the final forging temperature is 940℃, resulting in a forged bar with a diameter of 160mm.

[0075] Step S2: First annealing, anneal the first forging billet and air cool it to obtain the second forging billet;

[0076] After final forging, the first forged billet is placed in an annealing furnace for annealing at a temperature of 650℃ for 10 hours, and then air-cooled to room temperature.

[0077] Step S3: Normalizing. The second forging billet is normalized and then air-cooled to room temperature to obtain the third forging billet.

[0078] Normalizing temperature was 960℃, normalizing time was 4 hours, and then air-cooled to room temperature.

[0079] Step S4: Second annealing. The third forging billet is heated for annealing and then air-cooled to room temperature to obtain the fourth forging billet.

[0080] After the forging billet is normalized and then air-cooled to room temperature for 2 hours, it is annealed at 630℃ for 8 hours, and then air-cooled to room temperature.

[0081] Step S5: Quenching. The fourth forging billet is heated and held at a certain temperature for quenching treatment. Then it is cooled to room temperature by slow furnace cooling and oil cooling to obtain the fifth forging billet.

[0082] The holding temperature is 940℃, the holding time is 2 hours, and the furnace is slowly cooled to 850℃, and then oil cooled to room temperature.

[0083] Step S6: Tempering. The fifth forging billet undergoes a first tempering and a second tempering. The second tempering temperature is not higher than the first tempering temperature, resulting in multi-element microalloyed high-toughness ultra-high-strength steel.

[0084] The first tempering temperature was 240℃, and the tempering time was 5 hours. After air cooling to room temperature, the second tempering was carried out at 230℃ for 4 hours.

[0085] Example 2

[0086] The alloy composition of the raw steel billet in this embodiment is the same as that in Example 1, as shown in Table 1.

[0087] The preparation method of this embodiment is similar to that of Example 1, except for the process parameters and the addition of a cold treatment operation between the two tempering processes. The process parameters in the preparation process all meet the requirements of the present invention, as shown in Table 2.

[0088] The diameter of the forged rod is 300mm.

[0089] Example 3

[0090] The alloy composition of the raw steel billet in this embodiment, by weight percentage, is: C: 0.38%, Mn: 1.3%, Si: 1.4%, Cr: 1.25%, Ni: 0.20%, Mo: 0.5%, W: 0.3%, Nb: 0.008%, V: 0.005%, B: 0.0004%, with the balance being Fe and unavoidable impurities.

[0091] The preparation method in this embodiment is similar to that in Example 1, except for the process parameters during the preparation process; all process parameters during the preparation process meet the requirements of this invention, as shown in Table 2.

[0092] The diameter of the forged rod is 200mm.

[0093] Example 4

[0094] The alloy composition of the raw steel billet in this embodiment is the same as that in Example 3, as shown in Table 1.

[0095] The preparation method of this embodiment is similar to that of Example 1, except for the process parameters and the addition of a cold treatment operation between the two tempering processes. The process parameters in the preparation process all meet the requirements of the present invention, as shown in Table 2.

[0096] The diameter of the forged rod is 200mm.

[0097] Example 5

[0098] The alloy composition of the raw steel billet in this embodiment is the same as that in Example 3, as shown in Table 1.

[0099] The preparation method of this embodiment is similar to that of Example 1, except for the process parameters and the addition of a cold treatment operation between the two tempering processes. The process parameters in the preparation process all meet the requirements of the present invention, as shown in Table 2.

[0100] The diameter of the forged rod is 250mm.

[0101] Example 6

[0102] The alloy composition of the raw steel billet in this embodiment, by weight percentage, is: C: 0.36%, Mn: 1.8%, Si: 0.9%, Cr: 1.5%, Ni: 0.25%, Mo: 0.25%, W: 0.5%, Nb: 0.01%, V: 0.008%, B: 0.0008%, with the balance being Fe and unavoidable impurities.

[0103] The preparation method in this embodiment is similar to that in Example 1, except for the process parameters during the preparation process; all process parameters during the preparation process meet the requirements of this invention, as shown in Table 2.

[0104] The diameter of the forged rod is 80mm.

[0105] Example 7

[0106] The alloy composition of the raw steel billet in this embodiment, by weight percentage, is: C: 0.38%, Mn: 1.2%, Si: 1.3%, Cr: 1.12%, Ni: 0.4%, Mo: 0.36%, W: 0.15%, Nb: 0.005%, V: 0.006%, B: 0.0002%, with the balance being Fe and unavoidable impurities.

[0107] The preparation method in this embodiment is similar to that in Example 1, except for the process parameters during the preparation process; all process parameters during the preparation process meet the requirements of this invention, as shown in Table 2.

[0108] The diameter of the forged rod is 100mm.

[0109] Comparative Example 1

[0110] The alloy composition of the raw steel billet in this comparative example is the same as that in Example 1, as shown in Table 1.

[0111] The preparation method of this comparative example is similar to that of Example 1, except that the quenching process is not segmented cooling, but only tempering is performed once; as shown in Table 2.

[0112] The diameter of the forged rod is 160mm.

[0113] Comparative Example 2

[0114] The alloy composition of the raw steel billet in this comparative example is the same as that in Example 1, as shown in Table 1.

[0115] The preparation method of this comparative example is similar to that of Example 1, except for the process parameters during preparation; some process parameters during preparation do not meet the requirements of this invention, as shown in Table 2.

[0116] The diameter of the forged rod is 160mm.

[0117] Comparative Example 3

[0118] The alloy composition of the raw steel billet in this comparative example is the same as that in Example 3, as shown in Table 1.

[0119] The preparation method of this comparative example is similar to that of Example 1, except for the process parameters and the addition of a cold treatment operation between the two tempering processes; some process parameters in the preparation process do not meet the requirements of the present invention, as shown in Table 2.

[0120] The diameter of the forged rod is 250mm.

[0121] Comparative Example 4

[0122] The alloy composition of the raw steel billet in this comparative example, by weight percentage, is: C: 0.36%, Mn: 1.1%, Si: 1.2%, Cr: 1.21%, Ni: 0.25%, Mo: 0.11%, W: 0.1%, Nb: 0.003%, V: 0.003%, with the balance being Fe and unavoidable impurities. This does not meet the requirements of this invention.

[0123] The preparation method of this comparative example is similar to that of Example 1, except for the process parameters and the addition of a cold treatment operation between the two tempering processes, as shown in Table 2.

[0124] The diameter of the forged rod is 300mm.

[0125] Comparative Example 5

[0126] The alloy composition of the raw steel billet in this comparative example, by weight percentage, is: C: 0.30%, Mn: 1.0%, Si: 1.25%, Cr: 1.0%, Ni: 0.4%, Mo: 0.4%, W: 0.32%, V: 0.01%, with the balance being Fe and unavoidable impurities. This does not meet the requirements of this invention.

[0127] The preparation method of this comparative example is similar to that of Example 1, except for the process parameters during preparation, as shown in Table 2.

[0128] The diameter of the forged rod is 100mm.

[0129] Mechanical properties of the forged bars obtained in the examples and comparative examples were tested. The sample was taken at 1 / 2 of the cross-sectional radius of the bar. The material properties after the test are shown in Table 3.

[0130] Table 1. Alloy chemical composition (wt.%) of raw steel billets used in the examples and comparative examples.

[0131] project C Cr Ni Mn Mo Si W Nb V B Example 1 0.36 1.15 0.28 1.4 0.35 1.3 0.25 0.005 0.008 0.0003 Example 2 0.36 1.15 0.28 1.4 0.35 1.3 0.25 0.005 0.008 0.0003 Example 3 0.38 1.25 0.20 1.3 0.5 1.4 0.3 0.008 0.005 0.0004 Example 4 0.38 1.25 0.20 1.3 0.5 1.4 0.3 0.008 0.005 0.0004 Example 5 0.38 1.25 0.20 1.3 0.5 1.4 0.3 0.008 0.005 0.0004 Example 6 0.36 1.50 0.25 1.8 0.25 0.9 0.5 0.01 0.008 0.0008 Example 7 0.38 1.12 0.4 1.2 0.36 1.3 0.15 0.005 0.006 0.0002 Comparative Example 1 0.36 1.15 0.28 1.4 0.35 1.3 0.25 0.005 0.008 0.0003 Comparative Example 2 0.36 1.15 0.28 1.4 0.35 1.3 0.25 0.005 0.008 0.0003 Comparative Example 3 0.38 1.25 0.20 1.3 0.5 1.4 0.3 0.008 0.005 0.0004 Comparative Example 4 0.36 1.21 0.25 1.1 0.11 1.2 0.1 0.003 0.003 / Comparative Example 5 0.30 1.0 0.4 1.0 0.4 1.25 0.32 / 0.01 /

[0132] Table 2. Process parameters for the preparation of the examples and comparative examples.

[0133]

[0134]

[0135]

[0136] Table 3 Mechanical properties of the steels in the examples and comparative examples

[0137]

[0138] Figure 1A three-dimensional atomic probe microanalysis (APT) image of the austenite grain boundaries in the untempered state of Example 1 is shown. The image clearly shows an enrichment of Mo at the original austenite grain boundaries, along with small amounts of B and W enrichment.

[0139] Figure 2 The microstructure SEM image of Example 1 is shown. It can be seen that the microstructure of the material is mainly composed of lath martensite, and no obvious carbide white spots were observed between the laths, indicating that the precipitation of carbides was effectively controlled before tempering.

[0140] Figure 3 TEM images of the microstructure of Example 4 are shown. As can be seen from the images, the microstructure of the material consists of a matrix composed of martensite and thin-film retained austenite. Within the matrix, the martensite exhibits a pure state, and no coarse carbide structures were observed.

[0141] Figure 4 TEM images of the microstructure of Comparative Example 2 are shown. The images reveal that the microstructure of Comparative Example 2 is primarily composed of martensite and dense carbides, with the carbides being prominently distributed within the matrix.

[0142] Examples 1-7 all demonstrated good strength and toughness indicators. Among them, Example 1, for a general-sized forging (forging bar diameter 160mm), adopted a two-stage cooling process of slow furnace cooling + oil cooling during quenching and a two-stage tempering process during tempering, which resulted in superior strength and toughness performance.

[0143] Example 2 shows that for large-sized bar stock (forging bar diameter 300mm), a cold treatment operation was added during the tempering process, resulting in good strength and toughness performance. As can be seen from Examples 3 and 4, adding cold treatment during the tempering process can significantly improve the fracture toughness index of large-sized forgings.

[0144] The alloy compositions of Examples 3-5 use a slightly higher C content, but with the adjustment of slightly higher levels of grain boundary purifying elements such as Mo, W, and B, a balance between strength and toughness is achieved.

[0145] Example 6 slightly increased the content of Cr, Mn and W alloy components and adopted a two-stage cooling process of slow furnace cooling and oil cooling during quenching, which achieved good toughness while ensuring strength.

[0146] As can be seen from the comparison between Example 1 and Comparative Example 1, the preparation process of using a two-stage cooling method of slow cooling in the furnace and oil cooling during quenching, and a two-stage tempering process during tempering, can significantly improve the strength and toughness indicators.

[0147] As can be seen from the comparison between Example 1 and Comparative Example 2, the tempering temperature should not be set too high, otherwise it will affect both the strength and toughness of the material.

[0148] As can be seen from the comparison between Example 5 and Comparative Example 3, the two-stage quenching and cooling process of slow cooling + oil cooling can improve the overall strength and toughness of the material.

[0149] In Comparative Example 4, the content of Mo in the alloy composition does not meet the requirements of this invention, the content is too low, and there is no B, resulting in a significant decrease in material strength and a slight impact on toughness.

[0150] In Comparative Example 5, the C content in the alloy composition was too low, the Cr and Mn elements did not meet the requirements of this invention, and the Nb and B elements were not present, resulting in a significant decrease in material strength.

[0151] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A multi-element microalloyed high-toughness ultra-high-strength steel, characterized in that, The alloy composition of the multi-element microalloyed high-toughness ultra-high-strength steel, by weight percentage, is as follows: C: 0.36–0.38%, Mn: 1.10–2.0%, Si: 0.8–1.40%, Cr: 1.10–1.50%, Ni: 0.15–0.40%, Mo: 0.2–0.5%, W: 0.1–0.7%, Nb≤0.01%, V≤0.01%, B≤0.001%, with the balance being Fe and unavoidable impurities; The multi-element microalloyed high-toughness ultra-high-strength steel is prepared through the following steps: Step S1: Forging. The steel billet that meets the alloy composition requirements is heated in a soaking furnace and then forged to obtain the first forged billet. Step S2: First annealing, anneal the first forging billet and air cool it to obtain the second forging billet; Step S3: Normalizing. The second forging billet is normalized and then air-cooled to room temperature to obtain the third forging billet. Step S4: Second annealing. The third forging billet is heated for annealing and then air-cooled to room temperature to obtain the fourth forging billet. Step S5: Quenching. The fourth forging billet is heated and held at a certain temperature for quenching treatment. Then it is cooled to room temperature by slow furnace cooling and oil cooling to obtain the fifth forging billet. Step S6: Tempering. The fifth forging billet is subjected to a first tempering and a second tempering. The second tempering temperature is not higher than the first tempering temperature, to obtain multi-element microalloyed high-toughness ultra-high-strength steel. In step S5, the heat preservation temperature is 930℃~960℃ and the heat preservation time is 1~4h; the slow cooling and oil cooling process is as follows: the furnace is cooled to 830℃~860℃ and then oil cooled to room temperature. In step S6, the first tempering temperature is 235℃~255℃ and the tempering time is 3~5h; the second tempering temperature is 225℃~235℃ and the tempering time is 3~5h.

2. The multi-element microalloyed high-toughness ultra-high-strength steel according to claim 1, characterized in that, The microstructure of the multi-element microalloyed high-toughness ultra-high-strength steel consists of martensite and thin-film austenite.

3. The multi-element microalloyed high-toughness ultra-high-strength steel according to claim 2, characterized in that, The volume fraction of the austenite in the thin film is 3% to 7%.

4. A method for preparing a multi-element microalloyed high-toughness ultra-high-strength steel, characterized in that, The method for preparing the multi-element microalloyed high-toughness ultra-high-strength steel according to any one of claims 1-3 includes the following steps: Step S1: Forging. The steel billet that meets the alloy composition requirements is heated in a soaking furnace and then forged to obtain the first forged billet. Step S2: First annealing, anneal the first forging billet and air cool it to obtain the second forging billet; Step S3: Normalizing. The second forging billet is normalized and then air-cooled to room temperature to obtain the third forging billet. Step S4: Second annealing. The third forging billet is heated for annealing and then air-cooled to room temperature to obtain the fourth forging billet. Step S5: Quenching. The fourth forging billet is heated and held at a certain temperature for quenching treatment. Then it is cooled to room temperature by slow furnace cooling and oil cooling to obtain the fifth forging billet. Step S6: Tempering. The fifth forging billet undergoes a first tempering and a second tempering. The second tempering temperature is not higher than the first tempering temperature, resulting in multi-element microalloyed high-toughness ultra-high-strength steel.

5. The preparation method according to claim 4, characterized in that, In step S1, the initial forging temperature is 1050℃~1150℃, and the final forging temperature is 920℃~950℃.

6. The preparation method according to claim 5, characterized in that, In steps S2 and S4, the annealing temperature is 600~650℃ and the annealing time is 2~10h.

7. The preparation method according to claim 6, characterized in that, In step S3, the normalizing temperature is 950~980℃ and the normalizing time is 2~10h.

8. The preparation method according to claim 7, characterized in that, In step S5, the heat preservation temperature is 935℃~960℃, and the heat preservation time is 1~2h; The furnace slow cooling and oil cooling process is as follows: the furnace is cooled to 840℃~860℃ and then oil cooled to room temperature.

9. The preparation method according to claim 8, characterized in that, In step S6, the first tempering temperature is 240℃~255℃ and the tempering time is 3~5h; the second tempering temperature is 230℃~235℃ and the tempering time is 3~5h.

10. The preparation method according to claim 4, characterized in that, In step S6, a cold treatment operation is also included between the first tempering and the second two-step tempering operation.

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