A method for reducing the yield strength ratio of ultra-high strength maraging steel

Abstract

The application discloses a method for reducing the yield ratio of ultrahigh-strength maraging steel. The application arranges the I-type martensite laths parallel to the elongation direction and the II-type martensite laths at an angle of 40-60 degrees with the elongation direction through high-temperature rotary forging; the size of nanophase is reduced through solid solution and two-stage aging treatment, thereby improving the strength; and the inverse austenite generated in the two-stage aging process is used to improve the plasticity. The ultrahigh-strength maraging steel obtained has a microstructure of specific orientation martensite matrix + dispersed nanophase + inverse austenite, the yield ratio is less than 0.95, the tensile strength is 2136+28 MPa, the yield strength is 1945+23 MPa, and the elongation rate is 7.8+1%. The application realizes the low yield ratio while meeting the ultrahigh strength and excellent plasticity, increases the plasticity reserve, avoids the sudden failure of the material in the actual use process, and improves the service safety of the material.

Description

Technical Field

[0001] This invention belongs to the field of martensitic aging steel technology, specifically relating to a method for reducing the yield strength ratio of ultra-high strength martensitic aging steel. Background Technology

[0002] In 2022, my country's crude steel output reached 1.018 billion tons, exceeding half of the global output, making it a veritable steel superpower. However, the variety of steel produced is insufficient, the structure is unreasonable, and some high-quality, high-performance special steels still rely on imports. Martensitic aging steel is a special type of ultra-high-strength steel. It uses carbon-free or ultra-low-carbon iron-nickel martensite as a matrix. Through aging treatment, fine and dispersed intermetallic compounds precipitate in the lath martensite to achieve ultra-high strength. It features high strength, good ductility and toughness, simple heat treatment, and excellent weldability and cold and hot workability. It has been widely used in aerospace, machinery manufacturing, precision molds, automobiles, and other fields. However, according to literature reports ([1] He Yi et al. Grain size of ultrapure 18Ni martensitic aging steel and its influence on tensile properties [J]. Acta Metallurgica Sinica, 2002; [2] Bi Zhengxu. Influence of grain refinement on tensile properties of 18Ni(300) martensitic aging steel [J]. Special Steel Technology, 2014; [3] Luo Hong et al. Influence of grain size on mechanical properties of 18Ni martensitic aging steel [J]. Materials Science and Technology, 2000), the yield strength ratio of martensitic aging steel is generally greater than 0.95, and the yield strength ratio of some systems even exceeds 0.98.

[0003] A higher yield strength ratio means the material's yield strength is closer to its tensile strength. However, materials with high yield strength ratios have less plastic reserve, resulting in less plastic deformation before fracture and lower reliability. They are also more prone to sudden fracture under overload. To obtain a lower yield strength ratio, improvements are often made to the chemical composition and processing, thereby altering the material's microstructure, lattice distortion, and grain boundary characteristics to influence the final mechanical properties. In low-carbon steel, reducing dislocation strengthening, precipitation strengthening, or the proportion of small-angle grain boundaries are often employed to lower the yield strength ratio while maintaining a relatively constant tensile strength. Alternatively, increasing the volume fraction of the hardening phase can increase tensile strength while lowering the yield strength ratio.

[0004] Martensitic aging steel has a matrix of single martensitic laths without soft phases, therefore increasing the volume fraction of hardening phases cannot reduce the yield strength ratio. The martensitic laths contain high-density dislocations and numerous fine, dispersed precipitates. Reducing the dislocation density and the number of precipitates lowers the yield strength, thus affecting the overall mechanical properties. Therefore, reducing dislocation strengthening and precipitation strengthening effects cannot be used to lower the yield strength ratio. Currently, there are no publicly reported studies on reducing the yield strength ratio of martensitic aging steel, and methods used in low-carbon steel cannot be directly applied to martensitic aging steel. Therefore, reducing the yield strength ratio of martensitic aging steel is an urgent problem to be solved. This invention aims to reduce the yield strength ratio of ultra-high-strength martensitic aging steel by employing a method of high-temperature rotary forging + solution treatment + two-stage aging treatment. This method reduces the yield strength ratio of ultra-high-strength martensitic aging steel, increases plasticity reserves, and reduces the risk of direct failure. Summary of the Invention

[0005] To address the aforementioned problems, the present invention aims to provide a method for reducing the yield strength ratio of ultra-high strength martensitic aging steel.

[0006] A method for reducing the yield strength ratio of ultra-high strength maraging steel involves preparing ultra-high strength maraging steel alloy ingots through vacuum suspension melting, followed by homogenization heat treatment and high-temperature rotary forging to obtain type I and type II oriented martensitic laths. Subsequently, solution treatment and two-stage aging treatment are performed to finally obtain a microstructure of two oriented martensitic laths + fine dispersed precipitates + inverted austenite, resulting in maraging steel with a yield strength ratio of less than 0.95 and excellent strength-ductility combination.

[0007] Preferably, the homogenization heat treatment is performed by holding at 1000-1200℃ for 1-2 hours, followed by air cooling to room temperature. This homogenization heat treatment process can eliminate casting stress and reduce component segregation.

[0008] Preferably, the high-temperature rotary forging is performed by holding the temperature at 1000-1100℃ for 20-40 minutes, rotating 90° during each forging, and having a forging ratio of 6-9. High-temperature rotary forging can arrange the Type I martensitic laths parallel to the elongation direction and make the Type II martensitic laths form an angle of 40-60° with the elongation direction.

[0009] Preferably, the solution treatment is performed by holding the solution at 800-900°C for 1 hour and then air cooling. This solution treatment process allows a large number of alloying elements to dissolve into the matrix to form a supersaturated solid solution, which causes solid solution strengthening and prepares for subsequent aging.

[0010] Preferably, the first-stage aging treatment is carried out at 360–380℃ for 2 hours. After the first-stage aging treatment, the Mo-enriched phase is precipitated in the martensitic matrix, which provides nucleation sites for the Ni3Ti phase to precipitate in the second-stage aging.

[0011] Preferably, the second-stage aging treatment is performed at 470–490℃ for 3–5 hours. After the second-stage aging treatment, a large number of fine and dispersed Ni3Ti phases are precipitated near dislocations and at the interface between the Mo-rich phase and the matrix. The two-stage aging treatment prolongs the aging time, increases the content of inverted austenite, and improves plasticity.

[0012] Preferably, the excellent strength and plasticity refer to the tensile strength of the martensitic aging steel being 2136±28MPa, the yield strength being 1945±23MPa, and the elongation being 7.8±1%.

[0013] Compared with the prior art, the significant advantages of this invention are:

[0014] (1) The present invention proposes a method to reduce the yield strength ratio of ultra-high strength martensitic aging steel. While satisfying ultra-high strength and excellent plasticity, it achieves a low yield strength ratio (<0.95), which improves the material's ability to resist plastic deformation, increases plasticity reserve, avoids sudden failure of the material in actual use, and improves the service safety of the material.

[0015] (2) The present invention proposes a method for reducing the yield strength ratio of ultra-high strength martensitic aging steel. This method is simple, low in cost, has excellent comprehensive performance, and is universally applicable. Attached Figure Description

[0016] Figure 1 This is a process diagram for reducing the yield strength ratio of ultra-high strength martensitic aging steel.

[0017] Figure 2 These are microstructure diagrams of ultra-high strength martensitic aging steel with low yield strength ratio, where (a): martensitic lath morphology after high-temperature rotary forging, (b): microstructure after first-stage aging, (c): microstructure after second-stage aging, and (d): phase composition after second-stage aging.

[0018] Figure 3 This is the room temperature tensile stress-strain curve of the ultra-high strength martensitic aging steel in Example 1. Detailed Implementation

[0019] This invention employs high-temperature forging with 90° rotation each time to align Type I martensite laths parallel to the elongation direction, while Type II martensite laths form an angle of 40-60° with the elongation direction. At this angle, the Type II martensite laths possess a higher Schmidt factor, allowing dislocations to slide within them, thus increasing the ductility of the Type II martensite. Solution treatment combined with two-stage aging reduces the size of the nanophase, thereby increasing strength. Simultaneously, the inverse austenite generated during the two-stage aging process improves plasticity. The resulting ultra-high strength martensitic aged steel has a yield strength ratio of less than 0.95, a tensile strength of 2136±28 MPa, a yield strength of 1945±23 MPa, and an elongation of 7.8±1%.

[0020] Example 1

[0021] Combination Figure 1 This embodiment provides a process for reducing the yield strength ratio of ultra-high strength martensitic aging steel, including:

[0022] (1) The composition of ultra-high strength martensitic aging steel is: 18Ni-8.5Co-5Mo-0.7Ti-0.2Al (wt.%). Pure elements with a purity ≥99.99% are weighed using a balance with an error of ±0.005g. All raw materials are ultrasonically cleaned and dried with alcohol, and then subjected to high-vacuum suspension melting with a vacuum degree of 4×10 -3 Pa, using high-purity argon as the protective gas, was melted 5 times, with the ingot being turned over each time, to form a master alloy ingot, which was then held at 1100℃ for 2 hours for homogenization heat treatment.

[0023] (2) The homogenized heat-treated alloy ingot is heated to 1100℃ and held for 30 min, then forged into a square on a forging machine. A section is selected as the elongation direction, and the ingot is rotated 90° each time it is forged to a size of 30×30 mm. 2 The square cross section is forged to 10×10mm 2 The forging ratio reaches 9, reducing the cross-section by 88.9%, aligning the Type I martensitic laths parallel to the elongation direction, and the Type II martensitic laths at a 40-60° angle to the elongation direction. Figure 2 As shown in (a) in the figure.

[0024] (3) The forged sample was held at 835℃ for 1 hour and then air-cooled to room temperature. Then, it underwent first-stage aging at 370℃ for 2 hours and was air-cooled to room temperature to promote the precipitation of Mo-rich phases (such as...) in the martensitic matrix. Figure 2 As shown in (b), nucleation sites are provided for the Ni3Ti phase precipitation during the second-stage aging process; the second-stage aging process involves holding at 485℃ for 3.5h and then air-cooling to room temperature, which induces a large number of fine and dispersed Ni3Ti phases to precipitate near dislocations and at the interface between the Mo-rich phase and the matrix (as shown in (b)). Figure 2(as shown in (c)); the two-stage aging treatment extended the aging time and increased the content of inverted austenite, as shown in (c). Figure 2 As shown in (d) in the figure, plasticity is improved.

[0025] (4) The material after secondary aging is processed into standard tensile specimens, using 1×10 -4 s -1 The strain rate was used to test the quasi-static room temperature tensile properties of martensitic aged steel on a universal testing machine, such as... Figure 3 As shown, the results indicate that the tensile yield strength is 1945±23MPa, the tensile strength is 2136±28MPa, the elongation is 7.8±1%, and the yield strength ratio is 0.91~0.93. While satisfying the requirements of ultra-high strength and excellent plasticity and toughness, a low yield strength ratio is achieved, which increases the plasticity reserve and avoids sudden failure of the material in actual use.

[0026] Example 2

[0027] Using the same smelting process as in Example 1, the steel was subjected to high-temperature rotary forging at 1100℃ with a forging ratio of 6. The subsequent two-stage aging treatment of 835℃ / 1h / AC solution + 370℃ / 2h / AC + 485℃ / 3.5h / AC was the same as in Example 1. The quasi-static room temperature tensile properties of the martensitic aged steel were tested. The results showed that the tensile yield strength was 1952±26MPa, the tensile strength was 2084±34MPa, the elongation was 8.1±1%, and the yield strength ratio was 0.92~0.94.

[0028] Example 3

[0029] Using the same smelting process as in Example 1, the steel was subjected to high-temperature free forging at 1100℃. The subsequent two-stage aging treatment of 835℃ / 1h / AC solution + 370℃ / 2h / AC + 485℃ / 3.5h / AC was the same as in Example 1. The quasi-static room temperature tensile properties of the martensitic aged steel were tested. The results showed that the tensile yield strength was 1964±28MPa, the tensile strength was 2079±24MPa, the elongation was 8.4±1%, and the yield strength ratio was 0.93~0.94.

[0030] Comparative Example 1

[0031] Using the same smelting process as in Example 1, without high-temperature rotary forging, the subsequent two-stage aging treatment of 835℃ / 1h / AC solution + 370℃ / 2h / AC + 485℃ / 3.5h / AC was the same as in Example 1. The quasi-static room temperature tensile properties of the martensitic aged steel were tested. The results showed that the tensile yield strength was 1968±27MPa, the tensile strength was 2073±36MPa, the elongation was 8.03±1%, and the yield strength ratio was 0.94~0.96.

[0032] Comparative Example 2

[0033] Using the same smelting process as in Example 1, without high-temperature rotary forging, the steel was directly subjected to solution treatment at 835℃ / 1h / AC + aging treatment at 485℃ / 3.5h / AC. The quasi-static room temperature tensile properties of the martensitic aging steel were tested. The results showed that the tensile yield strength was 1975±35MPa, the tensile strength was 2065±25MPa, the elongation was 7.6±1%, and the yield strength ratio was 0.95~0.97.

Claims

1. A method for reducing the yield strength ratio of ultra-high strength martensitic aging steel, characterized in that, Ultra-high strength martensitic aging steel alloy ingots were prepared by vacuum suspension melting. After homogenization heat treatment, they were then subjected to high-temperature rotary forging to obtain martensitic laths with two orientations: type I and type II. Subsequently, solution treatment and two-stage aging treatment were carried out to obtain martensitic aging steel with a yield strength ratio of less than 0.95 and excellent strength-plasticity combination. This martensitic aging steel has a microstructure of two orientations of martensitic laths, fine dispersed precipitates, and inverted austenite. The high-temperature rotary forging process involves holding the temperature at 1000~1100℃ for 20~40 minutes, rotating 90° during each forging, and having a forging ratio of 6~9. The high-temperature rotary forging process causes the Type I martensitic laths to be arranged parallel to the elongation direction and the Type II martensitic laths to form an angle of 40~60° with the elongation direction. The first-level aging treatment is: aging at 360~380℃ for 2 hours; the second-level aging treatment is: aging at 470~490℃ for 3~5 hours. The composition of ultra-high strength martensitic aging steel by weight percentage is: 18Ni-8.5Co-5Mo-0.7Ti-0.2Al.

2. The method as described in claim 1, characterized in that, The homogenization heat treatment is as follows: holding at 1000~1200℃ for 1~2 hours, followed by air cooling to room temperature.

3. The method as described in claim 1, characterized in that, The solution treatment is performed by holding the solution at 800~900℃ for 1 hour, followed by air cooling.

4. The method as described in claim 1, characterized in that, The aforementioned excellent strength and plasticity refer to the tensile strength of the martensitic aging steel being 2136±28MPa, the yield strength being 1945±23MPa, and the elongation being 7.8±1%.

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