A method for preparing high-strength SF1900 steel bar

Abstract

The application relates to the technical field of steel forging, and particularly discloses a preparation method of high-strength SF1900 steel bar, which comprises the following steps: preparing SF1900 steel ingot, high-temperature homogenizing, forging and heat treatment; forging: three-up-three-down is carried out on the pretreated steel ingot obtained through high-temperature homogenizing; the heating temperature of the first heating is 1180-1220 DEG C; the heating temperature of the second heating is 1080-1120 DEG C; the heating temperature of the third heating is 1030-1050 DEG C; the upsetting deformation of each heating is greater than or equal to 50%; heat treatment: the fine-grained bar obtained through forging is normalized and tempered to obtain the high-strength SF1900 steel bar. The prepared SF1900 steel bar has a primary austenite grain size greater than or equal to 7 levels, and the ratio of the mechanical properties in the transverse direction to the longitudinal direction is 98-102%, which can fully meet the use requirements of the aero-engine shaft material.

Description

Technical Field

[0001] This application relates to the field of steel forging technology, specifically to a method for preparing high-strength SF1900 steel bars. Background Technology

[0002] With the continuous development of the aviation field, the performance improvement of aero engines, as the "heart" of aircraft, is crucial to the entire aviation industry. The efficient operation of aero engines depends on the high performance of various key components, and the performance of metallurgical materials directly determines the quality and reliability of these components. In the pursuit of higher flight speeds, longer ranges, and greater maneuverability, extremely stringent requirements are placed on the strength, toughness, and fatigue life of metallurgical materials. Improving material properties not only enhances the overall performance of aero engines but also strengthens the safety and reliability of aircraft, reducing maintenance costs and accident risks. Therefore, the continuous research and application of high-performance metallurgical materials has become one of the key factors in the development of the aviation field.

[0003] SF1900 steel is a high-purity, high-alloy-ratio, ultra-high-strength steel with Ni, Al, and Mo as the main strengthening elements. It is an important material for the shafts of next-generation high-performance aero-engines, meeting the requirements of high power, high speed, high stability, and reliability of aero-engines. However, when using existing steel bar forging processes to prepare SF1900 steel bars, there is a problem that dendrites cannot be effectively broken, resulting in an uneven internal structure of the material. This seriously affects the fatigue performance of components and makes it difficult to meet the increasingly stringent requirements of the aerospace industry for metallurgical materials in terms of strength, toughness, and fatigue life.

[0004] Therefore, there is an urgent need to develop a forging process for ultra-high strength SF1900 steel bars to obtain ultra-high strength steel bars with uniform transverse and longitudinal mechanical properties and high grain size, so as to meet the requirements of the new generation of high-performance aero-engine shaft materials. Summary of the Invention

[0005] In order to obtain ultra-high strength SF1900 steel bars with uniform transverse and longitudinal mechanical properties and high grain size, this application provides a method for preparing high strength SF1900 steel bars.

[0006] In a first aspect, this application provides a method for preparing high-strength SF1900 steel bars, employing the following technical solution: A method for preparing high-strength SF1900 steel bars includes the following steps: preparing SF1900 steel ingots, high-temperature homogenization, forging, and heat treatment; Forging: The pretreated steel ingot obtained by high-temperature homogenization is subjected to three upsetting and three drawing processes; wherein the heating temperature of the first upsetting is 1180-1220℃; the heating temperature of the second upsetting is 1080-1120℃; and the heating temperature of the third upsetting is 1030-1050℃; the upsetting deformation of each upsetting process is ≥50%; Heat treatment: The fine-grained bars obtained by forging are normalized and tempered to obtain high-strength SF1900 steel bars.

[0007] This application provides a method for preparing high-strength SF1900 steel bars. Through high-temperature diffusion, upsetting and forging, and heat treatment, it can reduce ingot segregation, refine the internal grain size of the steel, and ensure that the longitudinal and transverse mechanical properties of the steel bars are uniform and consistent. The grain size of the steel bars is better than grade 7.0.

[0008] In this application, the steel ingot is first homogenized at high temperature to make its internal structure more uniform. Then, a three-upsetting and three-drawing forging process is carried out with a gradually decreasing heating temperature, and the upsetting deformation in each heat is controlled to be ≥50%. On the one hand, this increases the forging ratio while effectively reducing segregation in the steel ingot and improving the uniformity of the transverse and longitudinal mechanical properties of the steel ingot, ensuring that the bar has good mechanical properties in all directions. On the other hand, it also refines and homogenizes the grain size of the steel. Finally, the fine-grained bar obtained by forging is normalized and tempered to further improve the microstructure and properties of the bar, ultimately obtaining a high-strength SF1900 steel bar with an original austenite grain size ≥7 and a transverse to longitudinal mechanical property ratio of 98-102%. This bar can meet the stringent requirements of the new generation of high-performance aero-engine shaft materials for bar grain size and mechanical properties, and is better adapted to the high-power, high-speed, high-stability, and high-reliability working environment of aero-engines.

[0009] SF1900 steel is a high-purity, high-alloy-ratio ultra-high-strength steel with Ni, Al, and Mo as the main strengthening elements. Compared with the existing NiCo series ultra-high-strength steels, it has lower impurity content and a narrower range of chemical composition control. The impurity element requirements are Ti≤0.01%, S≤0.0007%, P≤0.006%, O≤10ppm, N≤10ppm, and Al content 0.85-0.95%, which can better meet the requirements of high power, high speed, high stability, and reliability of aero engines.

[0010] Optionally, the single hammer reduction in the first drawing and forging process is 50-100mm, and the final forging temperature is ≥950℃; the single hammer reduction in the second drawing and forging process is 50-100mm, and the final forging temperature is 900-950℃; the single hammer reduction in the third drawing and forging process is 50-80mm, and the final forging temperature is 800-850℃.

[0011] Optionally, the specific steps of the high-temperature homogenization are as follows: heating the SF1900 steel ingot to 1200℃-1220℃ at a heating rate of ≤100℃ / h, and holding it at this temperature for 40h-45h to obtain the pretreated steel ingot.

[0012] Optionally, the specific steps of the high-temperature homogenization are as follows: SF1900 steel ingot is held at 600-650℃ for 1-3 hours; then the temperature is increased to 800-900℃ at a heating rate of ≤100℃ / h, and held at this temperature for 2-3 hours; then the temperature is increased to 1200℃-1220℃ at a heating rate of ≤100℃ / h, and held at this temperature for 40-45 hours to obtain the pretreated steel ingot.

[0013] Optionally, the normalizing treatment temperature is 900-1000℃ and the holding time is 8-10h; the tempering treatment temperature is 600-700℃ and the time is ≥25h.

[0014] Optionally, the normalizing temperature is 930-960℃, and the tempering temperature is 630-680℃.

[0015] In this application, normalizing and tempering the fine-grained bars obtained by forging improves the microstructure and transverse and longitudinal mechanical properties of the bars, resulting in smaller grain size and better uniformity. Through experimental research, the inventors of this application discovered that further controlling the normalizing and tempering temperatures within the aforementioned range yields bars with even better microstructure uniformity, maintaining the ratio of transverse to longitudinal mechanical properties within the range of 99-101%, thus better meeting the requirements for next-generation high-performance aero-engine shaft materials.

[0016] In some embodiments, the normalizing temperature can be 900-930℃, 900-950℃, 900-960℃, 900-1000℃, 930-950℃, 930-960℃, 930-1000℃, 950-960℃, 950-1000℃, or 960-1000℃. In one specific implementation, the normalizing temperature can also be 900°C, 930°C, 950°C, 960°C, or 1000°C.

[0017] In some embodiments, the tempering temperature can be 600-630℃, 600-650℃, 600-680℃, 600-700℃, 630-650℃, 630-680℃, 630-700℃, 650-680℃, 650-700℃, or 680-700℃. In one specific implementation, the tempering temperature can also be 600°C, 630°C, 650°C, 680°C, or 700°C.

[0018] Optionally, the diameter of the SF1900 steel ingot is 660 mm; the method for preparing the SF1900 steel ingot includes the following steps: vacuum induction melting and vacuum arc remelting.

[0019] Secondly, this application provides a high-strength SF1900 steel bar, which, by weight, comprises the following chemical composition: 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≤0.10%, Si≤0.10%, Cu≤0.05%, Ti≤0.01%, S≤0.0007%, P≤0.006%, O≤10ppm, N≤10ppm, with the balance being Fe.

[0020] Optionally, the high-strength SF1900 steel bar has a diameter of 190-420mm, an original austenite grain size of ≥7, and a ratio of transverse to longitudinal mechanical properties of 98-102%.

[0021] In summary, this application has the following beneficial effects: 1. This application provides a method for preparing high-strength SF1900 steel bars. The method firstly involves high-temperature homogenization of the steel ingot to reduce chemical composition segregation; then, through three-upsetting and three-drawing forging with controlled temperature at each stage, the internal temperature difference of the steel ingot can be reduced, ensuring the consistency of the transverse and longitudinal properties of the steel ingot; finally, through normalizing and tempering treatments, the microstructure and transverse and longitudinal mechanical properties of the bar can be further improved, ultimately obtaining high-strength SF1900 steel bars with an original austenite grain size ≥ 7 and a transverse to longitudinal mechanical property ratio of 98-102%.

[0022] 2. This application further controls the normalizing temperature at 930-960℃ and the tempering temperature at 630-680℃, resulting in SF1900 steel bars with better microstructure uniformity and the ratio of transverse to longitudinal mechanical properties can be maintained in the range of 99-101%. Detailed Implementation

[0023] This application provides a method for preparing high-strength SF1900 steel bars, comprising the following steps: (1) Preparation of steel ingots: (1-1) Vacuum induction melting: Prepare the raw 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; the raw materials should be clean, dry, oil-free, rust-free, and of accurate composition. 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. 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 100min 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. Casting: The refined molten steel is vacuum cast into steel ingots with a diameter of 580mm using the top pouring method. The casting temperature is controlled at 1540~1570℃. After casting, the mold is cooled for 120-180min to obtain the precast steel ingot. 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 560mm. (1-2) 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 5-6 kg / min, helium cooling is used, 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℃ at a rate of ≤100℃ / h, and held at the above temperature for ≥25 h, and then cooled to below 300℃ with the furnace, and air cooled to obtain an ultra-high strength SF1900 steel ingot with a diameter of 660 mm; (2) High temperature homogenization: SF1900 steel ingots are loaded into the furnace and held at 600-650℃ for 1-3 hours; then the temperature is increased to 800-900℃ at a rate of ≤100℃ / h and held at this temperature for 2-3 hours; then the temperature is increased to 1200-1220℃ at a rate of ≤100℃ / h and held at this temperature for 40-45 hours to obtain pretreated steel ingots; (3) Forging: The pretreated steel ingot is subjected to three upsetting and three drawing processes, as follows: First heating: Heat the pretreated steel ingot to 1180-1220℃, hold for 2-3 hours, then directly upset, upsetting deformation ≥50%, drawing and forging single hammer reduction 50-100mm, forging to 680mm octagon, final forging temperature ≥950℃, then return to furnace; Second heating: Continue heating to 1080-1120℃, hold for 1-3 hours, then directly upsetting, upsetting deformation ≥50%, drawing and forging single hammer reduction 50-100mm, forging to 680mm octagon, final forging temperature 900-950℃, then return to furnace; Third heating: Reheat to 1030-1050℃, hold for 1-3 hours, then directly upset, upsetting deformation ≥50%, drawing and forging single hammer reduction 50-80mm, final forging temperature 800-850℃, to obtain fine-grained bar with a diameter of 300mm; (4) Heat treatment: The fine-grained bar obtained by forging is air-cooled to 100-150℃, and then heated to 930-960℃ at a heating rate of ≤100℃ / h for normalizing treatment. The holding time of normalizing treatment is 8-10h, and then air-cooled to 100-150℃. Next, the bar is heated to 630-680℃ at a heating rate of ≤100℃ / h for tempering treatment. The holding time of tempering treatment is ≥25h, and then air-cooled to 20-30℃ to obtain a high-strength SF1900 steel bar with a diameter of 300mm.

[0024] The mass percentages of each component in the high-strength SF1900 steel bar are as follows: C: 0.14-0.25%, Mn: ≤0.10%, Si: ≤0.10%, 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%, Cu≤0.05%, S≤0.0007%, P≤0.006%, with the balance being Fe. All raw materials, reagents, and solvents used in this application are commercially available.

[0025] The present application will be further described in detail below with reference to embodiments and performance testing. Example 1

[0026] Example 1 provides a method for preparing high-strength SF1900 steel bars.

[0027] The preparation method of the above-mentioned high-strength SF1900 steel bar includes the following steps: (1) Vacuum induction melting: 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; 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. 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. 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. (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. (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. (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. After melting, helium is used for cooling, and the helium flow rate is controlled at 260 L / min. The cooling time 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. It is 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 SF1900 steel ingot with a height of 1.8 m and a diameter of 660 mm. (5) High temperature homogenization: The above SF1900 steel ingots are loaded into the furnace and heated to 1220℃ at a heating rate of 70℃ / h, and held at this temperature for 43h to obtain pretreated steel ingots; (6) Forging: The pretreated steel ingot is subjected to three upsetting and three drawing processes, as follows: First heating: The pretreated steel ingot is heated to 1180℃ and held for 2 hours. Then, the steel ingot is directly upset until the height of the steel ingot is 0.75m (the upsetting deformation is 58.3%). Then, it is drawn and forged. The single hammer reduction of the drawing and forging is 80mm, the final forging temperature is 975℃, and it is drawn into an octagonal near-round steel ingot with a diameter of 680mm and a height of 1.45m. Then it is returned to the furnace. Second heating: Continue heating to 1120℃, hold for 2 hours, then directly upset the steel ingot until the ingot height is 0.6m (upsetting deformation is 58.6%), then perform elongation forging, with a single hammer reduction of 80mm, and a final forging temperature of 930℃, elongating it into an octagonal near-round steel ingot with a diameter of 680mm and a height of 1.35m, and then return it to the furnace; Third heating: Reheat to 1050℃, hold for 2 hours, then directly upset the steel ingot until the height of the steel ingot is 0.55m (upsetting deformation is 59.2%), then perform elongation forging, with a single hammer reduction of 60mm and a final forging temperature of 870℃, elongating it into a fine-grained bar with a diameter of 300mm; (7) Heat treatment: The fine-grained bar obtained by forging is air-cooled to 120°C, and then heated to 950°C at a heating rate of 70°C / h for normalizing treatment. The holding time of normalizing treatment is 10h, and then air-cooled to 120°C. Next, the temperature is heated to 650°C at a heating rate of 70°C / h for tempering treatment. The holding time of tempering treatment is 30h, and then air-cooled to 25°C to obtain a high-strength SF1900 steel bar with a diameter of 300mm. Example 2

[0028] Example 2 provides a method for preparing high-strength SF1900 steel bars.

[0029] The difference between the above embodiment and embodiment 1 is that: (3) the deformation amount of upsetting in the forging step is as follows: (3) Forging: The pretreated steel ingot is subjected to three upsetting and three drawing processes, as follows: First heating: The pretreated steel ingot is heated to 1180℃ and held for 2 hours. Then it is directly upset until the height of the steel ingot is 0.9m (the upsetting deformation is 50.0%). Then it is drawn and forged. The single hammer reduction of the drawing and forging is 80mm. The final forging temperature is 975℃. It is drawn into an octagonal near-round steel ingot with a diameter of 680mm and a height of 1.5m and then returned to the furnace. Second heating: Continue heating to 1120℃, hold for 2 hours, then directly upsetting until the ingot height is 0.6m (upsetting deformation is 60.0%), then perform elongation forging, with a single hammer reduction of 80mm, and a final forging temperature of 930℃, elongating to an octagonal near-round steel ingot with a diameter of 680mm and a height of 1.4m, and then returning it to the furnace; Third heating: Reheat to 1050℃, hold for 2 hours, then directly upset until the ingot height is 0.55m (upsetting deformation is 60.7%), then perform elongation forging, with a single hammer reduction of 60mm and a final forging temperature of 870℃, elongating to a fine-grained bar with a diameter of 300mm. Example 3

[0030] Example 3 provides a method for preparing high-strength SF1900 steel bars.

[0031] The difference between the above embodiment and embodiment 1 is that: (3) the heating temperatures of the three heating cycles in the forging process are as follows: the heating temperature of the first heating cycle is 1200℃, the heating temperature of the second heating cycle is 1100℃, and the heating temperature of the third heating cycle is 1030℃. Example 4

[0032] Example 4 provides a method for preparing high-strength SF1900 steel bars.

[0033] The difference between the above embodiment and embodiment 1 is that: (3) the heating temperatures of the three heating cycles in the forging process are as follows: the heating temperature of the first heating cycle is 1190℃, the heating temperature of the second heating cycle is 1080℃, and the heating temperature of the third heating cycle is 1040℃. Example 5

[0034] Example 5 provides a method for preparing high-strength SF1900 steel bars.

[0035] The difference between the above embodiment and embodiment 1 is that: (2) the high-temperature homogenization step adopts a stepped heating and heat preservation method, as follows: (2) High temperature homogenization: The above SF1900 steel ingot is loaded into the furnace and held at 630℃ for 2 hours; then the temperature is increased to 850℃ at a heating rate of 70℃ / h and held at this temperature for 2.5 hours; then the temperature is increased to 1220℃ at a heating rate of 70℃ / h and held at this temperature for 43 hours to obtain the pretreated steel ingot.

[0036] Examples 6-9

[0037] Examples 6-9 provide a method for preparing high-strength SF1900 steel bars.

[0038] The difference between the above embodiments and Embodiment 5 lies in the temperature and time of the normalizing and tempering treatments, as shown in Table 1 below.

[0039] Table 1. Temperature and time of normalizing and tempering treatments in Examples 5-9 Comparative Example 1

[0040] Comparative Example 1 provides a method for preparing high-strength SF1900 steel bars.

[0041] The difference between the above comparative example and Example 1 is that: (3) the amount of deformation during upsetting in the forging step is as follows: (3) Forging: The pretreated steel ingot is subjected to three upsetting and three drawing processes, as follows: First heating: The pretreated steel ingot is heated to 1180℃ and held for 2 hours. Then it is directly upset until the height of the steel ingot is 1.2m (the upsetting deformation is 33.3%). Then it is drawn and forged. The single hammer reduction of the drawing and forging is 80mm. The final forging temperature is 975℃. It is drawn into an octagonal near-round steel ingot with a diameter of 680mm and a height of 1.45m, and then returned to the furnace. Second heating: Continue heating to 1120℃, hold for 2 hours, then directly upsetting until the ingot height is 0.9m (upsetting deformation is 37.9%), then perform elongation forging, with a single hammer reduction of 80mm, and a final forging temperature of 930℃, elongating to an octagonal near-round steel ingot with a diameter of 680mm and a height of 1.35m, then return to the furnace; Third heating: Reheat to 1050℃, hold for 2 hours, then directly upset until the ingot height is 0.8m (upsetting deformation is 40.7%), then perform elongation forging, with a single hammer reduction of 60mm and a final forging temperature of 870℃, elongating to a fine-grained bar with a diameter of 300mm. Comparative Example 2

[0042] Comparative Example 2 provides a method for preparing high-strength SF1900 steel bars.

[0043] The difference between the above comparative example and Example 1 is that (3) the heating temperature of the three heating cycles in the forging step is 1100℃. Comparative Example 3

[0044] Comparative Example 3 provides a method for preparing high-strength SF1900 steel bars.

[0045] The difference between the above comparative example and Example 1 is that: (3) the heating temperatures of the three heating cycles in the forging process are as follows: the heating temperature of the first heating cycle is 1220℃, the heating temperature of the second heating cycle is 1200℃, and the heating temperature of the third heating cycle is 1180℃. Comparative Example 4

[0046] Comparative Example 4 provides a method for preparing high-strength SF1900 steel bars.

[0047] The difference between the above comparative example and Example 1 is that: (3) the forging steps are two upsetting and two drawing, as follows: First heating: The pretreated steel ingot is heated to 1180℃ and held for 2 hours. Then it is directly upset until the height of the steel ingot is 0.9m (the upsetting deformation is 50.0%). Then it is drawn and forged. The single hammer reduction of the drawing and forging is 80mm. The final forging temperature is 975℃. It is drawn into an octagonal near-round steel ingot with a diameter of 680mm and a height of 1.5m and then returned to the furnace. Second heating: Reheat to 1180℃, hold for 2 hours, then directly upset until the ingot height is 0.55m (upsetting deformation is 60.7%), then perform elongation forging, with a single hammer reduction of 60mm and a final forging temperature of 870℃, elongating to a fine-grained bar with a diameter of 300mm.

[0048] Performance testing

[0049] The transverse tensile strength, transverse room temperature yield strength, longitudinal room temperature yield strength, edge grain size, and center grain size of the high-strength SF1900 steel bars obtained in Examples 1-9 and Comparative Examples 1-4 after final heat treatment were tested; the results are shown in Table 2 below.

[0050] Table 2 Performance test results of high-strength SF1900 steel bars obtained in Examples 1-9 and Comparative Examples 1-4

[0051] According to the test results in Table 2, the high-strength SF1900 steel bars obtained in Examples 1-9 of this application have a tensile strength ≥2000MPa, a ratio of transverse to longitudinal tensile strength of 98-102%, a reduction of area ≥53%, a ratio of transverse to longitudinal reduction of area of ​​98-102%, and a grain size grade of 7-8.5. This indicates that the forging process of this application employs a three-upsetting and three-drawing process, and controls the temperature and upsetting deformation of each heat treatment within a specific range. This reduces ingot segregation, improves the uniformity of transverse and longitudinal mechanical properties of the ingot, and yields SF1900 steel bars with excellent microstructure and mechanical properties.

[0052] In Comparative Example 1, when the upsetting deformation in the three forging passes was controlled to be 33.3-40.7% (<50%), the resulting SF1900 steel bar had a transverse tensile strength of only 1902 MPa, a longitudinal tensile strength of 2021 MPa, a transverse-to-longitudinal tensile strength ratio of 94.1%, a transverse reduction of area of ​​47.3%, a longitudinal reduction of area of ​​50.2%, a transverse-to-longitudinal tensile strength ratio of 94.22%, and a grain grade of only 6.5. Therefore, this demonstrates that adjusting the upsetting deformation in the three forging passes to below 50% leads to a decrease in the transverse mechanical properties of the steel bar and an increase in the difference between transverse and longitudinal properties.

[0053] In Comparative Example 2, the heating temperature of all three heating cycles was adjusted to 1100℃. The resulting SF1900 steel bar had a transverse tensile strength of 2012 MPa and a longitudinal tensile strength of only 2018 MPa, with a transverse-to-longitudinal tensile strength ratio of 99.7%. The transverse reduction of area was 52.1%, the longitudinal reduction of area was 49.8%, the transverse-to-longitudinal tensile strength ratio was 104.62%, and the grain grade was only 5.5-6.0. Therefore, it is demonstrated that adjusting the heating temperature of the three heating cycles in the forging process to isothermal will lead to a slight decrease in the microstructure, grain size, and mechanical properties of the steel bar.

[0054] In Comparative Example 3, when the heating temperatures of the three forging cycles were adjusted to 1220℃, 1200℃, and 1180℃, the resulting SF1900 steel bar exhibited a transverse tensile strength of only 1912 MPa and a longitudinal tensile strength of only 1930 MPa, with a transverse-to-longitudinal tensile strength ratio of 99.07%. The transverse reduction of area was 50.2%, the longitudinal reduction of area was 49.3%, the transverse-to-longitudinal tensile strength ratio was 101.83%, and the grain grade was only 4.0. Therefore, it is demonstrated that adjusting the heating temperatures of the three forging cycles outside a specific range results in steel bars with poor microstructure, grain size, and mechanical properties.

[0055] In Comparative Example 4, a two-forging and two-drawing process was used in the forging steps. The resulting SF1900 steel bar had a transverse tensile strength of only 1910 MPa and a longitudinal tensile strength of only 1927 MPa. The ratio of transverse to longitudinal tensile strength was 99.12%, the transverse reduction of area was 49.6%, the longitudinal reduction of area was 48.9%, the ratio of transverse to longitudinal tensile strength was 101.43%, and the grain grade was only 3.5-4.0.

[0056] Further comparison of the test results of Examples 5-9 shows that the ratio of transverse to longitudinal tensile strength of the SF1900 steel bar obtained in Example 6 is 98.87%, and the ratio of transverse to longitudinal reduction of area is 102.9%; the ratio of transverse to longitudinal reduction of area of ​​the SF1900 steel bar obtained in Example 9 is 102.44%; while the ratio of transverse to longitudinal tensile strength of the SF1900 steel bars obtained in Examples 5 and 7-8 is 99.71-100.24%, and the ratio of transverse to longitudinal reduction of area is 99.15-99.67%. This indicates that by further controlling the normalizing temperature at 930-960℃ and the tempering temperature at 630-680℃, the obtained SF1900 steel bar has better microstructure uniformity, and the ratio of transverse to longitudinal mechanical properties can be maintained within the range of 99-101%.

[0057] 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 preparing high-strength SF1900 steel bars, characterized in that, Includes the following steps: Preparation of SF1900 steel ingots, high-temperature homogenization, forging and heat treatment; High-temperature homogenization: SF1900 steel ingots are held at 600-650℃ for 1-3 hours; then the temperature is increased to 800-900℃ at a rate of ≤100℃ / h and held at this temperature for 2-3 hours; then the temperature is increased to 1200℃-1220℃ at a rate of ≤100℃ / h and held at this temperature for 40-45 hours to obtain pretreated steel ingots; Forging: The pretreated steel ingot obtained through high-temperature homogenization undergoes three upsetting and three drawing processes; the heating temperature of the first upsetting is 1180-1220℃, and the final forging temperature is ≥950℃; the heating temperature of the second upsetting is 1080-1120℃, and the final forging temperature is 900-950℃; the heating temperature of the third upsetting is 1030-1050℃, and the final forging temperature is 800-850℃; the upsetting deformation in each upsetting process is ≥50%; Heat treatment: The fine-grained bar obtained by forging is normalized and tempered to obtain high-strength SF1900 steel bar; the normalizing temperature is 930-960℃ and the holding time is 8-10h; the tempering temperature is 630-680℃ and the time is ≥25h. The high-strength SF1900 steel bar has a diameter of 190-420mm, an original austenite grain size of ≥7, a tensile strength of ≥2000MPa, and a ratio of transverse to longitudinal mechanical properties of 98-102%.

2. The method for preparing high-strength SF1900 steel bars according to claim 1, characterized in that, The single hammer reduction in the first drawing and forging process is 50-100mm; the single hammer reduction in the second drawing and forging process is 50-100mm; and the single hammer reduction in the third drawing and forging process is 50-80mm.

3. The method for preparing high-strength SF1900 steel bars according to claim 1, characterized in that, The method for preparing SF1900 steel ingots includes the following steps: vacuum induction melting and vacuum arc remelting.

4. A high-strength SF1900 steel bar obtained by the preparation method according to any one of claims 1-3, characterized in that, The chemical composition by weight is as follows: 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≤0.10%, Si≤0.10%, Cu≤0.05%, Ti≤0.01%, S≤0.0007%, P≤0.006%, O≤10ppm, N≤10ppm, with the balance being Fe.

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