Method for manufacturing ultra-high strength steel ingot and ultra-high strength steel bar

Abstract

The application relates to the metallurgical technical field, and particularly discloses a manufacturing method of ultrahigh-strength steel ingot, which comprises the following steps: vacuum induction smelting, casting, annealing and vacuum consumable remelting; the vacuum induction smelting comprises the following steps: batching, melting and refining; melting: materials except metal Al are added into a crucible in the order of adding industrial pure iron first and metal materials later, low vacuum is drawn to 20-90 Pa, the melting temperature is 1500-1550 DEG C, and the molten steel is obtained after melting; refining: then, the temperature is adjusted to 1520-1570 DEG C, the vacuum degree is less than or equal to 3 Pa, the molten steel is refined under the action of electromagnetic stirring for 30-120 min, metal Al is added into the molten steel after the O content in the molten steel is less than or equal to 10 ppm and the N content is less than or equal to 10 ppm, and the refined molten steel is obtained. The manufacturing method provided by the application can obtain ultrahigh-strength steel with low impurity content, no radial segregation and no defects such as ring pattern.

Description

Technical Field

[0001] This application relates to the field of metallurgical technology, specifically to a method for manufacturing ultra-high strength steel ingots and ultra-high strength steel bars. Background Technology

[0002] With the continuous development of the aerospace industry, the performance requirements for metallurgical materials are becoming increasingly stringent, especially in terms of strength, toughness, and fatigue life. To meet these requirements, it is essential to strictly control the content of impurity elements and non-metallic inclusions, as well as reduce the degree of material segregation. High-alloy-ratio NiCo series ultra-high-strength steels, such as AF1410 steel and A-100 steel, have been widely used in the field of aero-engine shaft materials because they can basically meet the above conditions.

[0003] Currently, for high-alloy-ratio NiCo series ultra-high-strength steels, the industry standard for impurity content control is as follows: AF1410 steel and A-100 steel both require Al ≤ 0.015%, Ti ≤ 0.015%, S ≤ 0.005%, P ≤ 0.008%, O ≤ 20ppm, and N ≤ 15ppm. However, the control of chemical composition and impurity content in these steels is not precise enough. The contents of impurities such as Ti, S, P, N, and O remain relatively high, and the steel ingots are prone to defects such as radial segregation and annular patterns. This makes it difficult to obtain ultra-high-strength steel bars with superior mechanical properties, thus failing to better meet the higher mechanical performance requirements of next-generation high-performance aero-engine shaft materials. Summary of the Invention

[0004] In order to further optimize and control the chemical composition and impurity content of ultra-high strength steel, and to solve the problems of radial segregation and annular patterns in steel ingots, so as to obtain ultra-high strength steel bars with better mechanical properties, this application provides a method for manufacturing ultra-high strength steel ingots and ultra-high strength steel bars.

[0005] Firstly, this application provides a method for manufacturing ultra-high strength steel ingots, employing the following technical solution:

[0006] A method for manufacturing ultra-high strength steel ingots includes the following steps: vacuum induction melting, casting, annealing, and vacuum arc remelting; wherein, the vacuum induction melting includes the following steps:

[0007] Ingredients: Prepare materials according to the following ratio: C 0.14-0.25%, Cr 1.5-3.0%, Ni 13.00-15.00%, Mo 1.00-2.00%, Co 9.00-11.00%, Al 0.85-0.95%, with the balance being industrial pure iron;

[0008] Melting: Materials other than metallic Al are added to the crucible in the order of first processing industrial pure iron and then adding the metal materials. The vacuum is evacuated to 20-90 Pa, and the melting temperature is 1500-1550℃. Molten steel is obtained after melting.

[0009] Refining: Next, adjust the temperature to 1520-1570℃ and the vacuum degree to ≤3Pa. Refine the molten steel for 30-120 minutes under electromagnetic stirring. When the O content and N content in the molten steel are ≤10ppm and ≤10ppm respectively, add metallic Al to the molten steel to obtain refined molten steel.

[0010] This application provides a method for manufacturing ultra-high strength steel ingots. The method uses industrial pure iron as the main raw material in the batching step, which reduces the content of impurity elements from the source, solving the problem that vacuum induction furnace smelting cannot remove impurity elements such as S, P, and Ti from steel. In the melting step, melting is carried out under a low vacuum of 20-90 Pa and a temperature of 1500-1550℃, effectively solving the problem of radial segregation and annular patterns in steel ingots. Then, in the refining step, refining is carried out under specific temperature, vacuum, and electromagnetic stirring, which effectively removes impurities and gases from the molten steel, bringing the O and N content to specific levels. Then, metallic Al is added, which not only avoids the risk of increased inclusions in high-Al steel but also refines the grains, further improving the strength and toughness of the steel. In summary, the vacuum induction melting step in the manufacturing method of ultra-high strength steel ingots provided in this application can provide high-quality refined molten steel for subsequent casting, annealing and vacuum arc remelting, laying a solid foundation for manufacturing high-purity, low-segregation ultra-high strength steel ingots.

[0011] The manufacturing method of ultra-high strength steel ingots provided in this application can strictly control non-metallic inclusions and impurity elements, such as S, P, Si, Mn, Ti, O, and N, to be at low levels, so that the low-magnification structure of the steel ingot reaches a high grade. For example, white spots, dark spots, radial segregation, and annular patterns all reach the AAAA level. Among the non-metallic inclusions, the A fine series is grade 0, the B fine series is grade 0, the C fine series is grade 0, and the D fine series is grade 0.5. The impurity elements are S < 0.0007%, P < 0.006%, Si ≤ 0.02%, Mn ≤ 0.02%, Ti < 0.01%, O ≤ 10ppm, and N ≤ 10ppm, thereby meeting the increasingly stringent performance requirements of metallurgical materials in the aerospace field and other fields.

[0012] Optionally, the chemical composition of the industrial pure iron, by mass content, is: C≤0.24%, S≤0.0007%, P≤0.004%, Si≤0.03%, Mn≤0.03%, Al≤0.010%, Ti<0.005%, Cu≤0.05%, O≤30ppm, N≤45ppm;

[0013] The industrial pure iron is processed as follows before use: the industrial pure iron is smelted in an electric furnace + LF + VD argon protection casting steel ingot, and after hot rolling and grinding, it is cut into 120×120mm×300mm blocks for later use.

[0014] Optionally, the specific steps of the casting are as follows: the refined steel obtained by vacuum induction melting is vacuum cast using the top pouring method, the casting temperature is controlled at 1540-1570℃, and after casting is completed, the mold is cooled for 120-180 minutes to obtain the precast steel ingot.

[0015] Optionally, the specific steps of the annealing are as follows: the precast steel ingot is heated to 640-670℃ at a rate of ≤100℃ / h, and held at the above temperature for ≥15h, then cooled to below 300℃ in the furnace, and then machined to obtain an electrode steel ingot with a diameter of 550-560mm.

[0016] Optionally, the specific steps of the vacuum arc remelting are as follows: the electrode steel ingot obtained in the annealing step is added to the vacuum arc remelting furnace for remelting; the average melting rate during the stabilization stage is 4.5-6.5 kg / min, and helium is used for cooling; the cooling time after melting is ≥75 min, the steel ingot is slowly cooled for ≥56 h, and then the temperature is raised to 640-670 °C at a rate of ≤100 °C / h, and held at the above temperature for ≥25 h, and then cooled with the furnace to below 300 °C, and air-cooled to obtain an ultra-high strength steel ingot.

[0017] Optionally, the helium flow rate is 200-300 L / min.

[0018] Secondly, this application provides an ultra-high strength steel ingot manufactured using the aforementioned method, comprising the following chemical composition by weight: C 0.14-0.25%, Cr 1.5-3.0%, Ni 13.00-15.00%, Mo 1.00-2.00%, Co 9.00-11.00%, Al 0.85-0.95%, Mn ≤0.02%, Si ≤0.02%, Cu ≤0.05%, S ≤0.0007%, P ≤0.006%, O ≤10ppm, N ≤10ppm, with the balance being Fe.

[0019] Optionally, the diameter of the ultra-high strength steel ingot is 660 mm.

[0020] Thirdly, this application provides an ultra-high strength steel bar, which is forged from the ultra-high strength steel ingot using a high-speed forging machine, and the ultra-high strength steel bar has a diameter ≤420mm.

[0021] In summary, this application has the following beneficial effects:

[0022] 1. This application provides a method for manufacturing ultra-high strength steel ingots, including vacuum induction melting, casting, annealing, and vacuum arc remelting steps. By adjusting the feeding sequence, controlling the temperature and vacuum degree in the melting step, and refining under specific temperature and vacuum degrees, this application can further reduce the content of impurity elements in ultra-high strength steel ingots, solve defects such as radial segregation and annular patterns in ultra-high strength steel ingots, and avoid the risk of increased inclusions in high-Al steel, further improving the strength and toughness of the steel, laying a solid foundation for manufacturing high-purity, low-segregation ultra-high strength steel.

[0023] 2. By controlling the impurity elements in industrial pure iron, this application can effectively solve the problem that vacuum induction furnace smelting cannot remove impurity elements such as S, P, and Ti from steel, resulting in ultra-high strength steel with lower S, P, and Ti impurity content, meeting the requirements for use.

[0024] 3. This application addresses the problem of radial segregation and circumferential patterns in existing high-alloy ratio ultra-high-strength steel ingots by controlling the process parameters in the vacuum arc remelting step, thereby obtaining steel ingots with high-grade microstructure at low magnification, such as white spots, dark spots, radial segregation, and circumferential patterns reaching AAAA level. Detailed Implementation

[0025] This application provides a method for manufacturing ultra-high strength steel ingots, comprising the following steps:

[0026] (1) Vacuum induction melting:

[0027] Ingredients: Prepare materials according to the following proportions: C 0.14-0.25%, Cr 1.5-3.0%, Ni 13.00-15.00%, Mo 1.00-2.00%, Co 9.00-11.00%, Al 0.85-0.95%, with the balance being industrial pure iron; raw materials should be clean, dry, oil-free, rust-free, and of accurate composition.

[0028] The chemical composition (wt%) of industrial pure iron is as follows: C≤0.24%, S≤0.0007%, P≤0.004%, Si≤0.03%, Mn≤0.03%, Al≤0.010%, Ti<0.005%, Cu≤0.05%, O≤30ppm, N≤45ppm. The above-mentioned industrial pure iron is smelted in an electric furnace + LF + VD furnace with argon protection and cast into steel ingots. After hot rolling and grinding, it is cut into 120×120mm×300mm blocks for later use.

[0029] Melting: Materials other than metallic Al are added to the crucible in the order of first processing industrial pure iron and then adding the metal materials. The vacuum is evacuated to 20-90 Pa, and the melting temperature is 1500-1550℃. Molten steel is obtained after melting.

[0030] Refining: Next, adjust the temperature to 1520-1570℃ and the vacuum degree to ≤3Pa. Refine the molten steel for 30-120 minutes under electromagnetic stirring. When the O content and N content in the molten steel are ≤10ppm and ≤10ppm respectively, add metallic Al to the molten steel to obtain refined molten steel.

[0031] (2) Casting: The refined molten steel is vacuum cast into steel ingots with a diameter of 580 mm by the top pouring method. The casting temperature is controlled at 1540-1570℃. After casting, the mold is cooled for 120-180 min to obtain the precast steel ingot.

[0032] (3) Annealing: The precast steel ingot is heated to 640-670℃ at a rate of ≤100℃ / h and held at the above temperature for ≥15h. Then it is cooled to below 300℃ in the furnace and polished to obtain an electrode steel ingot with a diameter of 550-560mm.

[0033] (4) Vacuum self-consuming remelting: The electrode steel ingot is added to the vacuum self-consuming furnace for remelting; the average melting rate during the smelting stabilization stage is 4.5-6.5 kg / min, helium is used for cooling, the helium flow rate is controlled at 200-300 L / min, and the cooling time after melting is ≥75 min; then the steel ingot is slowly cooled for ≥56 h, and then heated to 640-670 °C at a rate of ≤100 °C / h, and held at the above temperature for ≥25 h, and then cooled to below 300 °C with the furnace, and air-cooled to obtain an ultra-high strength steel ingot with a diameter of 660 mm.

[0034] The ultra-high strength steel ingot obtained by the manufacturing method of the ultra-high strength steel ingot provided in this application has the following chemical composition by weight: C 0.14-0.25%, Cr 1.5-3.0%, Ni 13.00-15.00%, Mo 1.00-2.00%, Co 9.00-11.00%, Al 0.85-0.95%, Mn ≤0.02%, Si ≤0.02%, Cu ≤0.05%, S ≤0.0007%, P ≤0.006%, O ≤10ppm, N ≤10ppm, and the balance is Fe.

[0035] This application provides an ultra-high strength steel bar, which is prepared by forging an ultra-high strength steel ingot using a 3500-ton high-speed forging machine to obtain a steel bar with a diameter ≤420mm.

[0036] The raw materials, reagents, solvents, etc. used in this application can all be obtained commercially.

[0037] The present application will be further described in detail below with reference to embodiments and performance testing. Example 1

[0038] Example 1 provides an ultra-high strength steel bar.

[0039] The preparation method of the above-mentioned ultra-high strength steel bar includes the following steps:

[0040] (1) Vacuum induction melting:

[0041] Ingredients: Prepare materials according to the following proportions: C 0.20%, Cr 2.46%, Ni 13.80%, Mo 1.47%, Co 10.00%, Al 0.85%, with the balance being industrial pure iron; raw materials should be clean, dry, oil-free, rust-free, and have accurate composition;

[0042] The chemical composition (wt%) of the industrial pure iron is as follows: C 0.17%, S 0.0007%, P 0.002%, Si 0.029%, Mn 0.022%, P 0.002%, Al 0.009%, Cu 0.02%, W 0.01%, V 0.01%, Ti 0.003%, O 24ppm, N 26ppm, with the balance being Fe. The above-mentioned industrial pure iron is smelted in an electric furnace + LF + VD argon-protected casting into steel ingots, which are then hot-rolled, ground, and cut into 120×120mm×300mm blocks for later use.

[0043] Melting: Materials other than metallic Al are added to the crucible in the order of first processing industrial pure iron and then adding the metal materials. The vacuum is reduced to 70 Pa and the melting temperature is 1510℃. Molten steel is obtained after melting.

[0044] Refining: Next, adjust the temperature to 1543℃ and the vacuum degree to 3Pa. Refine the molten steel for 100 minutes under electromagnetic stirring. The O content in the molten steel is 6ppm and the N content is 7ppm. Add metallic Al to the molten steel and adjust the chemical composition of Cr, Ni, Co, Mo and Al. Stir for 10 minutes to obtain refined molten steel.

[0045] (2) Casting: The refined molten steel is vacuum cast into steel ingots with a diameter of 580 mm by the top pouring method. The vacuum degree is <200 Pa and the pouring temperature is 1560℃. After casting, the mold is cooled for 150 min to obtain the precast steel ingot.

[0046] (3) Annealing: The precast steel ingot is heated to 650°C at a rate of 60°C / h and held at the above temperature for 15h. Then it is cooled to 300°C in the furnace and air-cooled. The annealed electrode steel ingot is then machined to have a smooth surface with metallic luster, no burrs and no grooves, to obtain an electrode steel ingot with a diameter of 560mm.

[0047] (4) Vacuum self-consuming remelting: The electrode steel ingot is added to the vacuum self-consuming furnace for remelting; the average melting rate during the smelting stabilization stage is 5 kg / min, helium is used for cooling, the helium flow rate is controlled at 260 L / min, and the cooling time after melting is 75 min; then the steel ingot is slowly cooled for 56 h, and then heated to 650 °C at a rate of 60 °C / h, and held at the above temperature for 25 h, and then cooled to 300 °C with the furnace and then air-cooled to obtain an ultra-high strength steel ingot with a diameter of 660 mm.

[0048] (5) Forging: The ultra-high strength steel ingot is forged by a 3500-ton high-speed forging machine to obtain a steel bar with a diameter of 300mm. Example 2

[0049] Example 2 provides an ultra-high strength steel bar.

[0050] The difference between the above embodiment and Embodiment 1 lies in the parameters of the melting and refining steps in vacuum induction melting, which are as follows:

[0051] Melting: Materials other than metallic Al are added to the crucible in the order of first processing industrial pure iron and then adding the metal materials. The vacuum is reduced to 20 Pa, and the melting temperature is 1530℃. Molten steel is obtained after melting.

[0052] Refining: Next, adjust the temperature to 1545℃ and the vacuum degree to 3Pa. Refine the molten steel for 100 minutes under electromagnetic stirring. The O content in the molten steel is 5ppm and the N content is 6ppm. Add metallic Al to the molten steel and adjust the chemical composition of Cr, Ni, Co, Mo and Al. Stir for 10 minutes to obtain refined molten steel. Example 3

[0053] Example 3 provides an ultra-high strength steel bar.

[0054] The difference between the above embodiment and Embodiment 1 lies in the parameters of the melting and refining steps in vacuum induction melting, which are as follows:

[0055] Melting: Materials other than metallic Al are added to the crucible in the order of first processing industrial pure iron and then adding the metal materials. The vacuum is evacuated to 90 Pa and the melting temperature is 1550℃. Molten steel is obtained after melting.

[0056] Refining: Next, adjust the temperature to 1570℃ and the vacuum degree to 3Pa. Refine the molten steel for 100 minutes under electromagnetic stirring. The O content in the molten steel is 8ppm and the N content is 9ppm. Add metallic Al to the molten steel and adjust the chemical composition of Cr, Ni, Co, Mo and Al. Stir for 10 minutes to obtain refined molten steel. Example 4

[0057] Example 4 provides an ultra-high strength steel bar.

[0058] The difference between the above embodiment and Embodiment 1 lies in the parameters of the vacuum arc remelting step, which are as follows:

[0059] Vacuum self-consumable remelting: The electrode steel ingot is added to a vacuum self-consumable furnace for remelting; the average melting rate during the stabilization stage is 4.5 kg / min. After melting, helium is used for cooling, with the helium flow rate controlled at 200 L / min and the cooling time at 75 min; then the steel ingot is slowly cooled for 56 h, and then heated to 670 °C at a rate of 80 °C / h, and held at the above temperature for 25 h. Then it is cooled to 300 °C in the furnace and air-cooled to obtain an ultra-high strength steel ingot with a diameter of 660 mm. Example

[0060] Example 5 provides an ultra-high strength steel bar.

[0061] The difference between the above embodiment and Embodiment 1 lies in the parameters of the vacuum arc remelting step, which are as follows:

[0062] Vacuum self-consumable remelting: The electrode steel ingot is added to a vacuum self-consumable furnace for remelting; the average melting rate during the stabilization stage is 6.5 kg / min. After melting, helium is used for cooling, with the helium flow rate controlled at 300 L / min and the cooling time at 75 min; then the steel ingot is slowly cooled for 56 h, and then heated to 640 °C at a rate of 70 °C / h, and held at the above temperature for 25 h. Then it is cooled to 300 °C in the furnace and air-cooled to obtain an ultra-high strength steel ingot with a diameter of 660 mm. Comparative Example 1

[0063] Comparative Example 1 provides an ultra-high strength steel bar.

[0064] The difference between the above comparative example and Example 1 lies in the parameters of the melting and refining steps in vacuum induction melting, which are as follows:

[0065] Melting: Materials other than metallic Al are added to the crucible in the order of first processing industrial pure iron and then adding the metal materials. The vacuum is reduced to 70 Pa, and the melting temperature is 1570℃. Molten steel is obtained after melting.

[0066] Refining: Next, adjust the temperature to 1543℃ and the vacuum degree to 3Pa. Refine the molten steel for 100 minutes under electromagnetic stirring. When the O content and N content in the molten steel are 20ppm and 20ppm respectively, add metallic Al to the molten steel and adjust the chemical composition of Cr, Ni, Co, Mo and Al. Stir for 10 minutes to obtain refined molten steel. Comparative Example 2

[0067] Comparative Example 2 provides an ultra-high strength steel bar.

[0068] The difference between the above comparative example and Example 1 is that the metallic Al was added at the early melting stage, simultaneously with all other materials, as detailed below:

[0069] Melting: Add all materials to the crucible in the order of first processing industrial pure iron and then adding metal materials, evacuate to a low vacuum of 20 Pa, melt at a melting temperature of 1530℃, and obtain molten steel after melting;

[0070] Refining: Next, adjust the temperature to 1545℃ and the vacuum degree to 3Pa. Refine the molten steel for 100 minutes under electromagnetic stirring. When the O content in the molten steel is 5ppm and the N content is 6ppm, adjust the chemical composition of Cr, Ni, Co, Mo and Al, and stir for 10 minutes to obtain refined molten steel. Comparative Example 3

[0071] Comparative Example 3 provides an ultra-high strength steel bar.

[0072] The difference between the above comparative example and Example 1 is that the vacuum arc remelting step is not included, as detailed below:

[0073] The method for preparing the ultra-high strength steel bar provided in Comparative Example 3 includes the following steps:

[0074] (1) Vacuum induction melting:

[0075] Ingredients: Prepare materials according to the following proportions: C 0.20%, Cr 2.46%, Ni 13.80%, Mo 1.47%, Co 10.00%, Al 0.85%, with the balance being industrial pure iron; raw materials should be clean, dry, oil-free, rust-free, and have accurate composition;

[0076] The chemical composition (wt%) of the industrial pure iron is as follows: C 0.17%, S 0.0007%, P 0.002%, Si 0.029%, Mn 0.022%, P 0.002%, Al 0.009%, Cu 0.02%, W 0.01%, V 0.01%, Ti 0.003%, O 24ppm, N 26ppm, with the balance being Fe. The above-mentioned industrial pure iron is smelted in an electric furnace + LF + VD argon-protected casting into steel ingots, which are then hot-rolled, ground, and cut into 120×120mm×300mm blocks for later use.

[0077] Melting: Materials other than metallic Al are added to the crucible in the order of first processing industrial pure iron and then adding the metal materials. The vacuum is reduced to 70 Pa and the melting temperature is 1510℃. Molten steel is obtained after melting.

[0078] Refining: Next, adjust the temperature to 1543℃ and the vacuum degree to 3Pa. Refine the molten steel for 100 minutes under electromagnetic stirring. The O content in the molten steel is 6ppm and the N content is 7ppm. Add metallic Al to the molten steel and adjust the chemical composition of Cr, Ni, Co, Mo and Al. Stir for 10 minutes to obtain refined molten steel.

[0079] (2) Casting: The refined molten steel is vacuum cast into steel ingots with a diameter of 580 mm by the top pouring method. The vacuum degree is 150 Pa and the pouring temperature is 1560℃. After casting, the mold is cooled for 150 min to obtain the precast steel ingot.

[0080] (3) Annealing: The precast steel ingot is heated to 650°C at a rate of 60°C / h and held at the above temperature for 15h. Then it is cooled to 300°C in the furnace and air-cooled. The annealed electrode steel ingot is then machined to have a smooth surface with metallic luster, no burrs and no grooves, to obtain an ultra-high strength steel ingot with a diameter of 560mm.

[0081] (4) Forging: The ultra-high strength steel ingot is forged by a 3500-ton high-speed forging machine to obtain a steel bar with a diameter of 300mm.

[0082] Performance testing

[0083] Chemical composition analysis was performed on the ultra-high strength steel bars of Examples 1-5 and Comparative Examples 1-3. The transverse low magnification structure was examined and rated according to ASTM A604-93. The content of non-metallic inclusions in the steel was detected and rated according to Method A in ASTM E 45. The results are shown in Table 1 below.

[0084] Table 1. Test results of various properties of ultra-high strength steel bars in Examples 1-5 and Comparative Examples 1-3.

[0085]

[0086]

[0087] According to the test results in Table 1, the main element content of the ultra-high strength steel bars obtained in Examples 1-5 meets the requirements, the content of impurity elements is lower than the specified value, and the white spots, dark spots, radial segregation and annular patterns in their low magnification structure reach AAAA level, and the non-metallic inclusions A, B, C and E are grade 0 and D are grade 0.5.

[0088] The main element content of the ultra-high strength steel bar obtained in Comparative Example 1 meets the requirements, but the contents of impurity elements O and N are both as high as 0.0012%, and the grades of non-metallic inclusions of types B, D, and E in the steel bar are relatively high. Therefore, it is evident that the ultra-high strength steel obtained by the parameters in the melting and refining steps of the vacuum induction melting process in Comparative Example 1 is unlikely to meet the higher requirements for steel in the aerospace field.

[0089] The main element content of the ultra-high strength steel bar obtained in Comparative Example 2 met the requirements. However, due to the high O and N content in the molten steel before refining, Al easily formed aluminum oxide or aluminum nitride inclusions with O and N, resulting in a high grade of non-metallic inclusions of types B and D in the steel bar. Therefore, this application demonstrates that by first adjusting the O and N content in the molten steel to a lower level during the refining step and then adding metallic Al, it can effectively avoid the formation of type B or D inclusions, thus ensuring the quality of the steel bar. Furthermore, it can refine the grains, further improving the strength and toughness of the steel.

[0090] The main element content of the ultra-high strength steel bar obtained in Comparative Example 3 met the requirements, but the Mn content of the impurity element was as high as 0.020%. The white spots, dark spots, radial segregation, and annular patterns in the low-magnification microstructure reached the AABB level, and the non-metallic inclusions in the steel bar, including categories B, D, and E, were of a relatively high grade. Therefore, it is indicated that the steel bar obtained in Comparative Example 3 without vacuum arc remelting has a high impurity content and defects such as radial segregation and annular patterns, which cannot meet the higher requirements for steel in the aerospace field.

[0091] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A method for manufacturing an ultra-high strength steel ingot, characterized in that, Includes the following steps: Vacuum induction melting, casting, annealing, and vacuum arc remelting; wherein, the vacuum induction melting includes the following steps: Ingredients: Prepare materials according to the following ratio: C 0.14-0.25%, Cr 1.5-3.0%, Ni 13.00-15.00%, Mo 1.00-2.00%, Co 9.00-11.00%, Al 0.85-0.95%, with the balance being industrial pure iron; The chemical composition of the industrial pure iron, by mass content, is as follows: C≤0.24%, S≤0.0007%, P≤0.004%, Si≤0.03%, Mn≤0.03%, Al≤0.010%, Ti≤0.01%, Cu≤0.05%, O≤30ppm, N≤45ppm; The industrial pure iron is processed as follows before use: the industrial pure iron is smelted in an electric furnace + LF + VD argon protection to cast steel ingots, and after hot rolling and grinding, it is cut into 120×120mm×300mm blocks for later use. Melting: Materials other than metallic Al are added to the crucible in the order of first processing industrial pure iron and then adding the metal materials. The vacuum is evacuated to 20-90 Pa, and the melting temperature is 1500-1550℃. Molten steel is obtained after melting. Refining: Next, adjust the temperature to 1520-1570℃ and the vacuum degree to ≤3Pa. Refine the molten steel for 30-120 minutes under electromagnetic stirring. When the O content and N content in the molten steel are ≤10ppm and ≤10ppm respectively, add metallic Al to the molten steel to obtain refined molten steel. The specific steps of the vacuum arc remelting are as follows: the electrode steel ingot obtained in the annealing step is added to the vacuum arc remelting furnace for remelting; the average melting rate during the smelting stabilization stage is 4.5-6.5 kg / min, and helium is used for cooling; the cooling time after melting is ≥75 min; the steel ingot is slowly cooled for ≥56 h, and then heated to 640-670 °C at a rate of ≤100 °C / h, and held at the above temperature for ≥25 h, and then cooled with the furnace to below 300 °C, and air-cooled to obtain an ultra-high strength steel ingot; The ultra-high strength steel ingot, by weight, comprises the following chemical components: Mn ≤0.02%, Si ≤0.02%, Cu ≤0.05%, S ≤0.0007%, P ≤0.006%, O ≤10ppm, N ≤10ppm.

2. The method for manufacturing ultra-high strength steel ingots according to claim 1, characterized in that, The specific steps of the casting are as follows: the refined steel obtained by vacuum induction melting is vacuum cast using the top pouring method, the casting temperature is controlled at 1540-1570℃, and after casting is completed, the mold is cooled for 120-180 minutes to obtain the precast steel ingot.

3. The method for manufacturing ultra-high strength steel ingots according to claim 1, characterized in that, The helium flow rate is 200-300 L / min.

4. The method for manufacturing ultra-high strength steel ingots according to claim 1, characterized in that, The specific steps of the annealing are as follows: the precast steel ingot is heated to 640-670℃ at a rate of ≤100℃ / h, and held at the above temperature for ≥15h. Then it is cooled to below 300℃ in the furnace and polished to obtain an electrode steel ingot with a diameter of 550-560mm.

5. The method for manufacturing ultra-high strength steel ingots according to claim 1, characterized in that, The diameter of the ultra-high strength steel ingot is 660 mm.

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