A medium-thickness 2200mpa-grade ultra-high strength steel and a preparation method thereof

Abstract

This invention discloses a medium-thickness 2200MPa grade ultra-high strength steel and its preparation method. The chemical composition by mass percentage is: C 0.40-0.42%, Si 1.0-1.1%, Mn 1.0-1.1%, Cr 1.0-1.1%, Ni 4.0-4.1%, Mo 0.30-0.35%, with the balance being iron and unavoidable impurities. This ultra-high strength steel adopts a low-alloy high-strength steel composition system, namely the Fe-C-Si-Mn-Cr-Ni-Mo composition system. Compared with martensitic aging steel and nano-bainitic steel, it does not contain the expensive element Co. At the same time, in order to ensure hardenability while giving the material certain weldability and low crack sensitivity, the designed C content is low, and the addition of Si, Mn, Cr and Mo is controlled as much as possible, so that the steel has the advantages of low cost and good weldability. The provided composition design gives the steel good hardenability. After rolling and heat treatment, when the steel plate thickness is 30-50mm, a martensitic matrix structure with a small amount of retained austenite can be obtained.

Description

Technical Field

[0001] This invention relates to the field of steel alloy materials technology, and in particular to a medium-thickness 2200MPa grade ultra-high strength steel and its preparation method. Background Technology

[0002] Ultra-high strength steel plates possess high strength, high hardness, and good impact toughness, effectively resisting the penetration of projectiles and fragments, and are widely used in protective engineering. However, they currently face development bottlenecks. Ultra-high strength steel plates with a strength of 2000MPa and above face a series of problems, including high alloy costs, complex manufacturing processes, and poor plasticity. Existing ultra-high strength steel plates with a strength of 2000MPa and above mainly include hot-stamped steel, nano-bainitic steel, high-carbon low-alloy steel, and medium-manganese steel. These materials generally require high alloying or complex processes to achieve high strength, and their relatively thin sheet thickness cannot meet the needs of special protective engineering projects.

[0003] Chinese invention patent CN112375990A discloses an ultra-high strength steel with a yield strength greater than 2000 MPa and its preparation method. Its chemical composition (by mass percentage) is: C: 0.2-0.4%, Mn: 6-9%, Si: 1-2%, V: 0.1-0.3%, with the balance being Fe and unavoidable impurities. Although this ultra-high strength steel achieves a tensile strength of approximately 2200 MPa and an elongation of over 10%, its excessively high Mn content leads to an excessively high C equivalent (the commonly used formula for calculating C equivalent is: CEV = C + Mn / 6 + Cr / 5 + Mo / 5 + V / 5 + Cu / 15 + Ni / 15), resulting in poor weldability. Furthermore, it employs a multi-pass forging process on the rotating billet or ingot to obtain the desired microstructure, which is impractical for large-scale steel plates required for protective engineering projects.

[0004] Chinese invention patent CN112981275A discloses a composite precipitation-strengthened steel with a strength of 2200 MPa or higher and its manufacturing method. Its chemical composition (by mass percentage) is: C: 0.32-0.40%, Ni: 11.0-16.0%, Co: 11.0-15.0%, Cr: 1.3-2.5%, Mo: 1.2-2.5%, Al: 0.2-2.0%, W: 0.2-1.2%, V: 0.01-0.2%, Nb: 0.01-0.20%, with the balance being Fe and unavoidable impurities. Although this strengthened steel achieves a tensile strength of over 2200 MPa, its alloy content is excessively high, and the required smelting method is vacuum induction melting + vacuum arc remelting, resulting in excessively high alloy and smelting costs. Furthermore, the low yield further increases the overall cost of the steel.

[0005] Chinese invention patent CN113930675A discloses a 2200MPa grade low-carbon, B-free hot-formed steel and its preparation method. Its chemical composition (by mass percentage) is: C: 0.30-0.35%, Si: ≤0.40%, Mn: 1.0-1.5%, Al: ≤0.05%, Cr: 0.2-0.3%, Mo: 0.15-0.25%, P: ≤0.008%, S≤0.005%, V: 0.03-0.05%, with the balance being Fe and unavoidable impurities. Although this hot-formed steel has an extremely high tensile strength of 2200MPa after hot forming, its elongation is only 5-6%, which is insufficient to meet the requirements for high elongation. Furthermore, the resulting steel is hot-rolled strip, which is relatively thin and cannot be used in heavy-duty protective engineering.

[0006] Chinese invention patent CN113957358A discloses a high-strength hot-formed steel substrate with a tensile strength greater than 2200 MPa and its preparation method. Its chemical composition by mass percentage is: C: 0.40-0.44%, Si: 0.35-0.44%, Mn: 1.4-1.5%, Cr: 1.9-2.4%, P: ≤0.01%, S≤0.005%, rare earth element Y: 0.015-0.055%, Nb: 0.045-0.06%, V: 0.15-0.20%, where V / Nb = 2.5-4.5, and the balance is Fe and unavoidable impurities. Although this hot-formed steel achieves a tensile strength exceeding 2200 MPa, its elongation does not exceed 9%, and it is a cold-rolled steel strip with relatively thin dimensions, making it unsuitable for heavy-duty protective engineering.

[0007] Chinese invention patent application CN111304537A discloses a 2200MPa grade prestressed steel strand and its production process. Its chemical composition (mass percentage) is: C: 0.84-0.92%, Si: 0.1-1.3%, Mn: 0.3-0.9%, Cr: 0.1-0.5%, P: ≤0.015%, S: ≤0.010%, Al: 0.01-0.08%, V: 0.01-0.10%, with the balance being Fe and unavoidable impurities. Although this steel strand achieves a strand strength of 2200-2300MPa, its excessively high C content leads to poor weldability. Furthermore, as it is a prestressed steel strand, strain hardening during the drawing process is its primary strengthening effect, offering no reference value for the development of medium-thickness 2200MPa grade ultra-high strength steel plates. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a medium-thickness 2200MPa grade ultra-high strength steel with good mechanical properties; the present invention also provides a method for preparing medium-thickness 2200MPa grade ultra-high strength steel.

[0009] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: its chemical composition by mass percentage is: C 0.40-0.42%, Si 1.0-1.1%, Mn 1.0-1.1%, Cr 1.0-1.1%, Ni 4.0-4.1%, Mo 0.30-0.35%, with the balance being iron and unavoidable impurities.

[0010] Furthermore, the thickness of the ultra-high strength steel is 30-50 mm, the tensile strength is >2150 MPa, and the elongation after fracture is >10%.

[0011] The functions of the main elements in this invention are as follows:

[0012] C: Add 0.40-0.42% C to obtain an ultra-high strength martensitic structure, which has high tensile strength, but does not excessively affect weldability and crack sensitivity.

[0013] Si: Si can improve the hardenability of steel and has a strong solid solution strengthening effect, which can significantly improve the tensile strength of steel. However, excessive Si content will significantly reduce the weldability of steel. Therefore, the Si content should be controlled between 1.0% and 1.1%.

[0014] Mn: Mn can refine the microstructure and improve hardenability, but excessive content will reduce the weldability of steel. Therefore, the Mn content should be controlled between 1.0% and 1.1%.

[0015] Cr: Cr can significantly improve the hardenability and tensile strength of steel. However, excessive Cr content will reduce the weldability, plasticity, and toughness of steel. Therefore, the Cr content should be controlled between 1.0% and 1.1%.

[0016] Ni: Ni can improve the hardenability of steel, while also increasing its strength and maintaining good plasticity and toughness. Compared with Cr and Mn, it has a smaller impact on the weldability of steel. However, Ni alloys are relatively expensive, so the Ni content is controlled at 4.0% to 4.1%.

[0017] Mo: Mo can refine grains and improve the hardenability of steel, but too high a content will reduce the weldability of steel. At the same time, Mo alloys are expensive, so the Mo content is controlled between 0.30 and 0.35%.

[0018] The method of the present invention includes rolling and heat treatment steps;

[0019] The rolling process involves heating and holding the billet or steel ingot at a specific temperature, and then rolling it into a sheet. During the rolling process, the rolling ratio is ≥8, the deformation per pass is >15%, the deformation in the final pass is ≥20%, and the final rolling temperature is 850-900℃.

[0020] The heat treatment steps are as follows: a quenching and tempering process is adopted; in the quenching process, the plate is heated to 850-870℃, held at the temperature, and then water-cooled; in the tempering process, the plate is heated to 175-185℃, held at the temperature, and then air-cooled; thus, the ultra-high strength steel plate can be obtained.

[0021] Furthermore, in the rolling step, the billet or ingot is heated to 1150–1200°C and held at that temperature.

[0022] Furthermore, in the heat treatment step, the plate is heated to 850-870℃ and held for 0.5-1.0h; the plate is heated to 175-185℃ and held for 1.0-2.0h.

[0023] The beneficial effects of adopting the above technical solution are as follows: This invention uses a low-alloy high-strength steel composition system, namely the Fe-C-Si-Mn-Cr-Ni-Mo system, which, compared to martensitic aging steel and nano-bainitic steel, does not add expensive Co elements. Simultaneously, to ensure hardenability while maintaining certain weldability and low crack sensitivity, the designed C content is low, and the addition amounts of Si, Mn, Cr, and Mo are controlled as much as possible, giving the steel the advantages of low cost and good weldability. The composition design provided by this invention gives the steel good hardenability, and after rolling and heat treatment, a martensitic matrix structure with a small amount of retained austenite can be obtained when the steel plate thickness is 30-50 mm. The ultra-high strength steel plate provided by this invention has excellent performance; that is, when the thickness reaches 30-50 mm, the mechanical properties can still reach: tensile strength > 2150 MPa, elongation after fracture > 10%, which can be used for various defensive fortifications and civil security facilities with extremely high requirements for steel plate thickness and strength.

[0024] This invention employs a low-alloy high-strength steel composition system with a low carbon content. While ensuring hardenability, it controls the addition of Si, Mn, Cr, and Mo as much as possible, without adding expensive Co, giving the steel the advantages of low cost and good weldability. Simultaneously, a simple controlled rolling method is used to achieve homogenization and refinement of the microstructure; combined with a quenching + low-temperature tempering heat treatment process, the mechanical properties of the steel plate are controlled. Even with a steel plate thickness of 30-50mm, the mechanical properties can still achieve: tensile strength > 2150MPa, elongation after fracture > 10%. It can be used for various defensive fortifications and civilian security facilities with extremely high requirements for steel plate thickness and strength. It features a simple process flow and good process controllability, making it suitable for medium and heavy plate production lines in major steel mills, enabling large-scale mass production. Attached Figure Description

[0025] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0026] Figure 1 This is a metallographic diagram of the ultra-high strength steel plate obtained in Example 1 of the present invention. Detailed Implementation

[0027] The chemical composition (by weight percentage) of this medium-thickness 2200MPa grade ultra-high strength steel is: C 0.40–0.42%, Si 1.0–1.1%, Mn 1.0–1.1%, Cr 1.0–1.1%, Ni 4.0–4.1%, Mo 0.30–0.35%, with the balance being iron and unavoidable impurities. The preparation method of this ultra-high strength steel includes smelting, rolling, and heat treatment steps.

[0028] (1) Smelting: Smelting according to the above chemical composition, casting into billets or steel ingots with a thickness of 250-600 mm.

[0029] (2) Rolling steps: Heat the billet or steel ingot to 1150-1200℃ and keep it at that temperature for 3-10 hours, and then roll it to obtain a plate with a thickness of 30-50mm. During the rolling process, control the rolling ratio ≥8, and the deformation amount of each pass >15%, the deformation amount of the final pass ≥20%, and control the final rolling temperature at 850-900℃. After rolling, the plate is air-cooled to room temperature.

[0030] (3) Heat treatment steps: First, a quenching process is carried out, followed by a tempering process; in the quenching process, the plate is heated to 850-870℃, held for 0.5-1.0h, and then water-cooled to room temperature; in the tempering process, the plate is heated to 175-185℃, held for 1.0-2.0h, and then air-cooled to room temperature; the ultra-high strength steel plate can then be obtained.

[0031] (4) The thickness of the obtained ultra-high strength steel plate is 30-50 mm, the tensile strength is >2150 MPa, the elongation after fracture is >10%, and the internal structure is martensite + a small amount of retained austenite.

[0032] Example 1: The medium-thickness 2200MPa grade ultra-high strength steel and its preparation method are described in detail below.

[0033] (1) After smelting, it is cast into a steel ingot with a thickness of 400mm. The chemical composition by mass percentage is: C 0.41%, Si 1.05%, Mn 1.05%, Cr 1.05%, Ni 4.05%, Mo 0.33%, with the balance being iron and unavoidable impurities.

[0034] (2) The 400mm thick steel ingot is heated to 1170℃ and held for 6 hours, and then rolled to obtain a plate with a thickness of 40mm, so that the rolling ratio is 10. The rolling process is shown in Table 1.

[0035] Table 1: Rolling Process Parameters

[0036]

[0037] The final rolling temperature is controlled at 870℃, and the rolled plate is air-cooled to room temperature.

[0038] (3) Heat the plate to 860°C, keep it at that temperature for 0.8h, and then cool it to room temperature with water; then heat it to 180°C, keep it at that temperature for 1.5h, and then air cool it to room temperature; and you can obtain a 40mm thick 2200MPa grade ultra-high strength steel plate.

[0039] (4) According to mechanical property testing, the performance indicators of the 40mm thick ultra-high strength steel plate prepared in this embodiment are: tensile strength 2190MPa, elongation after fracture 11.8%. Microstructure analysis shows that the obtained ultra-high strength steel plate has a microstructure of martensite + a small amount of retained austenite, such as... Figure 1 As shown.

[0040] Example 2: The medium-thickness 2200MPa grade ultra-high strength steel and its preparation method are described in detail below.

[0041] (1) After smelting, it is cast into a 250mm thick billet with the following chemical composition by mass percentage: C 0.40%, Si 1.00%, Mn 1.01%, Cr 1.00%, Ni 4.01%, Mo 0.30%, with the balance being iron and unavoidable impurities.

[0042] (2) The billet is heated to 1150℃ and held for 3 hours before rolling to obtain a plate with a thickness of 30mm, with a rolling ratio of 8.3. The rolling process is shown in Table 2.

[0043] Table 2: Rolling Process Parameters

[0044]

[0045] The final rolling temperature is controlled at 850℃, and the rolled plate is air-cooled to room temperature.

[0046] (3) Heat the plate to 850°C, keep it warm for 0.5h, and cool it with water to room temperature; then heat it to 175°C, keep it warm for 1.0h, and air cool it to room temperature; and you can get a 30mm thick 2200MPa grade ultra-high strength steel plate.

[0047] (4) According to the mechanical property test, the performance index of the 30mm thick ultra-high strength steel plate prepared in this embodiment is: tensile strength 2175MPa, elongation after fracture 12.0%. According to the microstructure test, the microstructure of the obtained ultra-high strength steel plate is martensite + a small amount of retained austenite.

[0048] Example 3: The medium-thickness 2200MPa grade ultra-high strength steel and its preparation method are described in detail below.

[0049] (1) After smelting, it is cast into a steel ingot with a thickness of 600mm. Its chemical composition by mass percentage is: C 0.42%, Si 1.1%, Mn 1.1%, Cr 1.1%, Ni 4.09%, Mo 0.35%, with the balance being iron and unavoidable impurities.

[0050] (2) Heat the steel ingot to 1200℃, hold it for 10 hours, and then roll it to obtain a plate with a thickness of 50mm. The rolling ratio is 12. The rolling process is shown in Table 3.

[0051] Table 3: Rolling Process Parameters

[0052]

[0053] The final rolling temperature is controlled at 900℃, and the rolled plate is air-cooled to room temperature.

[0054] (3) Heat the plate to 870°C, keep it warm for 1.0h, and cool it with water to room temperature; then heat it to 185°C, keep it warm for 2.0h, and air cool it to room temperature; and you can get a 50mm thick 2200MPa grade ultra-high strength steel plate.

[0055] (4) According to the mechanical property test, the performance index of the 50mm thick ultra-high strength steel plate prepared in this embodiment is: tensile strength 2200MPa, elongation after fracture 12.1%. According to the microstructure test, the microstructure of the obtained ultra-high strength steel plate is martensite + a small amount of retained austenite.

Claims

1. A medium-thickness, 2200MPa grade ultra-high strength steel, characterized in that, Its chemical composition by mass percentage is: C 0.40-0.42%, Si 1.0-1.1%, Mn 1.0-1.1%, Cr 1.0-1.1%, Ni 4.0-4.1%, Mo 0.30-0.35%, with the balance being iron and unavoidable impurities; the thickness of the ultra-high strength steel is 30-50 mm, the tensile strength is >2150 MPa, the elongation after fracture is >10%, and the internal structure is martensite + a small amount of retained austenite.

2. The method for preparing the medium-thickness 2200MPa grade ultra-high strength steel according to claim 1, characterized in that: This includes smelting, rolling, and heat treatment steps; The smelting steps are as follows: smelting according to chemical composition, casting into billets or steel ingots, with a billet or steel ingot thickness of 250-600 mm; The rolling process involves heating the billet or steel ingot to 1150–1200℃ and holding it at that temperature for 3–10 hours, then rolling it into a sheet with a thickness of 30–50 mm. During the rolling process, the rolling ratio is ≥8, the deformation per pass is >15%, the deformation in the final pass is ≥20%, and the final rolling temperature is 850–900℃. The heat treatment steps are as follows: a quenching and tempering process is adopted; in the quenching process, the plate is heated to 850-870℃ and held for 0.5-1.0h, and then water-cooled; in the tempering process, the plate is heated to 175-185℃ and held for 1.0-2.0h, and then air-cooled; thus, the ultra-high strength steel plate can be obtained.

3. A method of producing a medium thickness 2200 MPa grade ultra-high strength steel according to claim 2, characterized in that: In the rolling step, the rolled sheet is air-cooled to room temperature.

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