Method for reducing transverse and longitudinal performance difference of ultrahigh-strength steel material

Abstract

The invention relates to the field of steel processing, and discloses a method for reducing transverse and longitudinal performance difference of an ultrahigh-strength steel material. The method comprises the steps that a cast ingot with set components is smelted through vacuum induction, and then a vacuum consumable ingot is manufactured; the steel ingot is heated and subjected to heat preservation till primary carbide is fully redissolved; extruding the steel ingot until the ratio of the perimeter to the height of the cross section of the steel billet reaches a preset ratio, and reshaping; the blank is subjected to primary hot rolling alternately with the length direction and the width direction as the rolling direction; and the blank is subjected to secondary hot rolling alternately with the length direction and the width direction as the rolling direction till the thickness of the blank meets the requirement. According to the method, the alloy components and the hot working technology of the ultrahigh-strength steel are comprehensively regulated and controlled, the transverse and longitudinal performance difference of the ultrahigh-strength steel is effectively reduced, the average value ratio of the transverse and longitudinal tensile strength of the ultrahigh-strength steel is 99% or above, the average value ratio of the transverse and longitudinal yield strength of the ultrahigh-strength steel is 98% or above, and the average value ratio of the transverse and longitudinal U-opening impact toughness of the ultrahigh-strength steel is 100% or above.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of steel processing, in particular to a method for reducing the difference between transverse and longitudinal performance of super high strength steel material. BACKGROUND

[0002] With the continuous development of aerospace industry, the application scale of super high strength steel gradually increases. Compared with other steel materials, super high strength steel has a super high strength of 1500 MPa or more and high toughness, and is easy to process and heat treat, and is suitable for application in working environment with high strength requirement. Therefore, the application of super high strength steel in aerospace structural parts (such as solid rocket engine shell) has great significance. The steel for solid rocket engine shell is a key structural material for bearing extreme working conditions, which uses super high strength steel materials such as D406A, 30Cr3, T250, D6AC, etc. to improve the engine mass ratio and reduce the negative mass.

[0003] To comply with the development of the manufacturing level of the new generation of aerospace solid rocket engine, higher requirements are put forward for the performance of the steel for solid rocket engine metal shell. After rolling, the grains of the previous high strength steel (such as D406A, D6AC) extend along the rolling direction (longitudinal direction), forming a banded structure, and the transverse plasticity is significantly reduced, which is easy to cause micro-cracks, so that the shell material bears the combined stress of circumferential tensile and radial compression during spinning forming, and the circumferential (transverse) bears the main tensile stress, which exposes the short board of transverse plasticity.

[0004] Therefore, how to reduce the difference between transverse and longitudinal performance of super high strength steel material has become a technical problem to be solved in the field. SUMMARY

[0005] In view of the above problems existing in the prior art, the main purpose of the present application is to provide a method for reducing the difference between transverse and longitudinal performance of super high strength steel material.

[0006] According to one aspect of the present application, a method for reducing the difference between transverse and longitudinal performance of super high strength steel material is provided, which comprises the following steps: A casting ingot is smelted by a vacuum induction smelting method, and the casting ingot comprises the following components in percentage by mass: C: 0.32%~0.38%, Mo: 0.4%~0.8%, Ni: 3.5%~4.5%, Si: 1.0%~2.5%, Mn: 0.4%~0.8%, Nb: 0.010%~0.020%, Cr and V, S: 5~15ppm, O: 5~15ppm, N: 5~15ppm, and the balance is Fe and inevitable impurities, wherein Ni: Cr = 2~5: 1, V: Nb = 20~70: 1; The casting ingot is smelted into a vacuum consumable ingot by a vacuum consumable smelting method; The vacuum consumable ingot is heated and kept at a temperature of 1150-1180 DEG C for a period of time; The vacuum consumable ingot is extruded along the length direction to a ratio of the cross section perimeter to the height of the billet of 8-9:1. The billet is pressed along the thickness direction, and the length direction and the width direction perpendicular to the thickness direction are alternately taken as the rolling direction for primary hot rolling until the thickness variation of the billet meets the requirement. The billet is heated and then subjected to secondary hot rolling, in which the billet is pressed along the thickness direction, and the length direction and the width direction are alternately taken as the rolling direction, and the reduction rate of each pass of the secondary hot rolling is different from that of each pass of the primary hot rolling.

[0007] According to one embodiment of the present application, the vacuum consumable ingot is heated to 1150-1180 DEG C for keeping.

[0008] According to one embodiment of the present application, the keeping time is calculated based on the diameter of the vacuum consumable ingot and determined by the following formula: t=k*D, wherein t is the keeping time in min, k is the keeping coefficient related to the content of Mo and Nb in the material in min / mm, and D is the diameter of the vacuum consumable ingot in mm.

[0009] According to one embodiment of the present application, the vacuum consumable ingot is extruded along the length direction to a ratio of the cross section perimeter to the height of the billet of 8-9:1.

[0010] According to one embodiment of the present application, the thickness, width and length of the billet suitable for rolling are in a ratio of (1-1.5):1:(1-1.5).

[0011] According to one embodiment of the present application, the primary hot rolling includes 5-8 passes, the ratio of the initial thickness to the final thickness of the billet is 2.2-2.8:1, and the billet needs to be rotated by 90 DEG after each pass.

[0012] According to one embodiment of the present application, the reduction rate of each pass of the primary hot rolling is controlled as follows: the reduction rate of the first pass is 7-12%, the reduction rate of the intermediate pass is 14-21%, and the reduction rate of the last pass is 17-20%.

[0013] According to one embodiment of the present application, the secondary hot rolling includes 5-7 passes, the ratio of the initial thickness to the final thickness of the billet is 2.5-3.0:1, and the billet needs to be rotated by 90 DEG after each pass.

[0014] According to one embodiment of the present invention, the rolling direction of the first pass of the secondary hot rolling is perpendicular to the rolling direction of the last pass of the primary hot rolling.

[0015] According to one embodiment of the present invention, the reduction rate of each pass of the secondary hot rolling is controlled as follows: the reduction rate of the first pass is 4% to 6%, the reduction rate of the intermediate passes is 10% to 15%, and the reduction rate of the last pass is 12% to 14%.

[0016] Compared with the prior art, the method of the present invention for reducing the difference in transverse and longitudinal properties of ultra-high strength steel has at least one of the following beneficial effects: The method of the present invention effectively reduces the difference in transverse and longitudinal properties of medium and low alloy ultra-high strength steel by comprehensively controlling the alloy composition and hot working process of ultra-high strength steel. Its average longitudinal tensile strength is ≥1940MPa, average longitudinal yield strength is ≥1530MPa, longitudinal U-shaped impact strength is ≥50J, the ratio of average transverse and longitudinal tensile strength is above 99%, the ratio of average transverse and longitudinal yield strength is above 98%, and the ratio of average transverse and longitudinal U-shaped impact toughness is above 100%. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 A flowchart illustrating a method for mitigating the difference in transverse and longitudinal properties of ultra-high strength steel materials according to an embodiment of the present invention is shown. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0020] Furthermore, the reference to "embodiment" herein means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0021] It should be understood that the embodiments of the invention shown in the exemplary embodiments are merely illustrative. Although only a few embodiments have been described in detail in this invention, those skilled in the art will readily recognize that various modifications are possible without substantially departing from the teachings of the invention. Accordingly, all such modifications should be included within the scope of the invention. Other substitutions, modifications, variations, and deletions can be made to the design, operating conditions, and parameters of the following exemplary embodiments without departing from the spirit of the invention.

[0022] According to one aspect of the present invention, a method for mitigating the difference in transverse and longitudinal properties of ultra-high strength steel materials is provided, such as... Figure 1 As shown, the method mainly includes the following steps: Step S100: An ingot is produced by vacuum induction smelting. The ingot comprises the following components by mass percentage: C: 0.32%~0.38%, Mo: 0.4%~0.8%, Ni: 3.5%~4.5%, Si: 1.0%~2.5%, Mn: 0.4%~0.8%, Nb: 0.010%~0.020%, Cr and V, S: 5~15ppm, O: 5~15ppm, N: 5~15ppm, with the balance being Fe and unavoidable impurities, wherein Ni:Cr = 2~5:1, V:Nb = 20~70:1; Step S200: The ingot is smelted into a vacuum consumable ingot through vacuum consumable metallurgy; Step S300: Heat and hold the vacuum consumable ingot until the Nb-V composite primary carbide is fully dissolved; Step S400: Extrude the vacuum consumable ingot along the length direction until the ratio of the cross-sectional perimeter to the height of the billet reaches a predetermined ratio, and then shape it into a billet suitable for rolling; Step S500: Press the billet down along the thickness direction, and alternately use the length direction and width direction perpendicular to the thickness direction as the rolling direction for one hot rolling until the thickness change of the billet meets the requirements; Step S600: After heating the billet, it is subjected to secondary hot rolling. In the secondary hot rolling, the billet is pressed down along the thickness direction, and the length direction and width direction are alternately used as the rolling direction. The reduction rate of each pass of the secondary hot rolling is different from that of each pass of the primary hot rolling.

[0023] The method of this invention effectively reduces the difference in transverse and longitudinal properties of medium and low alloy ultra-high strength steel by comprehensively controlling the alloy composition and hot working process of ultra-high strength steel. Its average longitudinal tensile strength is ≥1940MPa, average longitudinal yield strength is ≥1530MPa, and longitudinal U-shaped impact toughness is ≥50J. The ratio of the average transverse and longitudinal tensile strength is above 99%, the ratio of the average transverse and longitudinal yield strength is above 98%, and the ratio of the average transverse and longitudinal U-shaped impact toughness is above 100%.

[0024] The following is an exemplary description of each step of the above method.

[0025] In step S100, an ingot is produced by vacuum induction smelting. The ingot comprises the following components by mass percentage: C: 0.32%~0.38%, Mo: 0.4%~0.8%, Ni: 3.5%~4.5%, Si: 1.0%~2.5%, Mn: 0.4%~0.8%, Nb: 0.010%~0.020%, Cr and V, S: 5~15ppm, O: 5~15ppm, N: 5~15ppm, with the balance being Fe and unavoidable impurities, wherein Ni:Cr = 2~5:1, V:Nb = 20~70:1.

[0026] The ingots of this invention require a Ni:Cr ratio of 2~5:1. This is to optimize the strength and plasticity of the steel, especially its plasticity and toughness, so that it does not crack during subsequent hot deformation with large deformation. Furthermore, this method can produce ultra-high-strength steel with a toughness level of 1900MPa. Ni is a strong austenite stabilizing element, which can lower the temperature of martensite to austenite reversal during tempering, providing a thermodynamic driving force for the formation of inverted austenite. This makes it easier for inverted austenite to precipitate during tempering, while retaining a certain amount of residual austenite to increase plasticity. Inverted austenite precipitates along the boundaries of martensite laths, finely and dispersed between the martensite laths. During crack propagation, inverted austenite can effectively delay crack propagation and increase the energy required for crack propagation. Cr is a ferrite-forming element, which shrinks the austenite phase region. To stabilize the austenite structure, it needs to be balanced with austenite-forming elements such as nickel (Ni). Therefore, the Ni:Cr ratio needs to be controlled within a certain range to obtain inverted austenite and retained austenite, so that the steel can obtain sufficient plasticity and toughness.

[0027] The ingots of this invention require a V:Nb ratio of 20~70:1. Adding Nb to steel can refine the grains, but Nb and C easily form large NbC particles, and NbC has a high remelting temperature of 1250~1280℃. This necessitates heating to a high temperature during subsequent hot deformation to ensure sufficient NbC remelting. The method of this invention adds V to the steel. V preferentially combines with C in the steel to form fine VC precipitates. This process consumes some C, inhibiting the formation of coarse NbC. Simultaneously, Nb grows around the VC particles, forming Nb-V composite carbides. The remelting temperature of these carbides is only 1130~1180℃, significantly lower than that of NbC. Simultaneously adding Nb and V and controlling their ratio within the range set by this invention achieves grain refinement and strengthening while allowing primary carbides to precipitate dispersedly, reducing transverse and longitudinal differences.

[0028] The vacuum induction smelting method in this step can be implemented using conventional methods in the field.

[0029] In step S200, the ingot is smelted into a vacuum consumable ingot through vacuum arc remelting. This step can be carried out using methods conventional in the art. For example, after peeling and trimming the ends of a vacuum induction ingot, it can be smelted into a vacuum consumable ingot through vacuum arc remelting. Vacuum arc remelting can improve the purity of the steel ingot, reduce segregation, and obtain a steel ingot with highly uniform composition and microstructure, thus providing a basic guarantee for subsequent hot working.

[0030] In step S300, the vacuum consumable ingot is heated and held at a temperature until the Nb-V composite primary carbide is fully dissolved. During this process, the Nb-V composite primary carbide is fully dissolved, and the elements are allowed to diffuse sufficiently, causing the carbides to precipitate diffusely during cooling, resulting in a uniformly dispersed microstructure. This avoids the precipitation of banded carbides and prevents the formation of carbide streaks in the material. A uniformly dispersed microstructure reduces the differences in strength, plasticity, and toughness between the transverse and longitudinal directions of the steel.

[0031] In some embodiments, the vacuum consumable ingot is heated to 1150~1180°C and held at that temperature to allow the Nb-V composite primary carbide to fully dissolve.

[0032] In some embodiments, the holding time depends on the diameter of the vacuum consumable ingot and is determined by the following formula: t=k*D, Where t is the heat preservation time in min; k is the heat preservation coefficient, which is related to the content of Mo and Nb elements in the material and is in min / mm. When (Mo+Nb)≤2%, k is 6~7, and when (Mo+Nb)>2%, k is 7~8; D is the diameter of the vacuum consumable ingot in mm.

[0033] In step S400, the vacuum consumable ingot is extruded along its length until the ratio of the circumference to the height of the billet cross-section reaches a predetermined ratio, and then shaped into a billet suitable for rolling. "Bill cross-section" refers to the section perpendicular to the extrusion direction, and "height" refers to the direction parallel to the extrusion direction. This step employs a large deformation to provide recrystallization driving force and increase nucleation sites, promoting dynamic recrystallization and thus refining the grain size. Simultaneously, the composition and microstructure of the steel become more uniform under the influence of the large deformation, reducing the difference between the transverse and longitudinal properties of the steel. Due to the large deformation in this operation, the ingot requires high plasticity. As mentioned above, to ensure that the plasticity of the ingot meets the requirements of this processing technology, the ingot composition needs a certain content and proportion of Ni and Cr elements (Ni:Cr = 2~5:1) to improve austenite stability. At the same time, the ratio of Nb and V (V:Nb = 20~70:1) must be strictly controlled to control the size of the carbides and prevent them from becoming brittle phases that deteriorate processing performance. The addition of manganese (Mn) is to form MnS with sulfur (S) in the steel, preventing the formation of iron sulfide which causes "hot brittleness" in the steel. In some embodiments, the vacuum consumable ingot is extruded along its length until the ratio of the cross-sectional perimeter to the height of the billet reaches 8-9:1. Large deformation is used to refine the grains and make the composition more uniform.

[0034] In some embodiments, after a set deformation amount is reached, the steel billet is shaped into a billet suitable for rolling. For example, the thickness, width and length ratio of the billet is (1~1.5):1:(1~1.5).

[0035] In step S500, the billet is pressed down along the thickness direction, and hot rolling is performed once by alternately using the length direction and width direction perpendicular to the thickness direction as the rolling direction, until the thickness change of the billet meets the requirements.

[0036] After shaping the steel billet into a suitable rolling material, the billet undergoes annealing and then multi-pass alternating rolling at the initial rolling temperature. When the length direction is used as the rolling direction, the billet extends along its length while maintaining a constant width as it is rolled down. When the width direction is used as the rolling direction, the billet extends along its width while maintaining a constant length as it is rolled down. Conventional rolling operations only roll in a single direction, resulting in significant differences in transverse and longitudinal properties. This invention alternates the rolling direction during rolling, thereby reducing the differences in transverse and longitudinal properties.

[0037] In some embodiments, a single hot rolling operation comprises 5 to 8 passes, with the initial thickness to final rolling thickness ratio of the billet being 2.2 to 2.8:1. After each pass, the billet is rotated 90°. The initial thickness of the billet refers to its thickness before rolling. Controlling the initial thickness to final rolling thickness ratio to 2.2 to 2.8:1 ensures sufficient deformation of the billet, thereby refining the grain size and microstructure. Grain refinement effectively enhances both the strength and toughness of the steel.

[0038] In some embodiments, the reduction rate of each pass in a single hot rolling process is controlled as follows: the reduction rate for the first pass is 7% to 12%, the reduction rate for the intermediate passes is 14% to 21%, and the reduction rate for the last pass is 17% to 20%. By comprehensively controlling the number of rolling passes and the reduction rate, the ratio of the initial thickness to the final rolled thickness of the billet is 2.2 to 2.8:1.

[0039] During this process, because the billet contains carbon (0.32%~0.38%), nitrogen (0.010%~0.020%), and v, and the ratio of nitrogen to v is strictly controlled (V:Nb=20~70:1), fine and uniformly distributed NbC, VC, and their composite carbides will be dispersed and precipitated under the aforementioned rolling deformation. These carbides, either dissolved during rolling heating or precipitated by strain during rolling, can pin grain boundaries and dislocations, hindering the growth and recrystallization of austenite grains, thereby refining the microstructure and grain size. Simultaneously, after tempering, these dispersed fine carbides (especially nanoscale carbides) produce a strong precipitation strengthening effect and can increase the matrix hardness, significantly improving the strength, hardness, and wear resistance of the steel.

[0040] In step S600, the billet is heated and then subjected to secondary hot rolling. In the secondary hot rolling, the billet is pressed down along the thickness direction, and the length direction and width direction are alternately used as the rolling direction. The reduction rate of each pass of the secondary hot rolling is different from that of each pass of the primary hot rolling.

[0041] In some embodiments, the secondary hot rolling includes 5 to 7 passes, with the initial thickness to final rolling thickness ratio of the billet being 2.5 to 3.0:1. After each pass, the billet needs to be rotated 90°. The initial thickness of the billet refers to the thickness before rolling after the forming is completed.

[0042] In some embodiments, the rolling direction of the first pass of the secondary hot rolling is perpendicular to the rolling direction of the last pass of the primary hot rolling. The alternating rolling directions of different passes reduce the difference in transverse and longitudinal properties.

[0043] In some embodiments, the reduction rate of each pass in the secondary hot rolling is controlled as follows: the reduction rate of the first pass is 4% to 6%, the reduction rate of the intermediate passes is 10% to 15%, and the reduction rate of the last pass is 12% to 14%.

[0044] During this process, steel containing the components of this invention forms a fine and uniform grain structure under the aforementioned rolling deformation, which means less stress concentration and more uniform deformation capacity, significantly improving the plastic forming limit of the steel. Carbides gradually transform from lath-like particles into more uniform, finer spherical particles. This transformation effectively eliminates stress concentration while increasing the strength of the material.

[0045] After the above processing and heat treatment, the microstructure of the ultra-high strength steel is lath martensite, finely dispersed carbides, and a certain amount of retained austenite and reverse-transformed austenite.

[0046] The method of this invention effectively reduces the difference in transverse and longitudinal properties of medium and low alloy ultra-high strength steel. The average longitudinal tensile strength is ≥1940MPa, the average longitudinal yield strength is ≥1530MPa, the average longitudinal U-shaped impact strength is ≥50J, the average ratio of transverse and longitudinal tensile strength is ≥99%, the average ratio of transverse and longitudinal yield strength is ≥98%, and the average ratio of transverse and longitudinal U-shaped impact toughness is ≥100%.

[0047] To more clearly and in detail describe the method of mitigating the difference in transverse and longitudinal properties of ultra-high strength steel materials according to the present invention, specific embodiments will be described below.

[0048] Example 1 Vacuum induction smelting ingot, the ingot comprising the following components by mass percentage: C: 0.36%, Mo: 0.7%, Ni: 3.8%, Si: 1.4%, Mn: 0.6%, Cr: 0.95%, V: 0.75%, Nb: 0.015%, S: 6ppm, O: 8ppm, N: 9ppm, with the balance being Fe and unavoidable impurities, wherein Ni:Cr = 4:1, V:Nb = 50:1.

[0049] After peeling and cutting off the top and bottom of the ingot, it is smelted into a vacuum consumable ingot through vacuum self-consumption.

[0050] The vacuum consumable ingot was heated to 1180℃ and held to allow the Nb-V composite primary carbide to fully dissolve. The ingot diameter was 250mm, Mo+Nb≤1.2%, k was set to 7, and the holding time was 1750min.

[0051] The vacuum consumable ingot is extruded along its length until the ratio of the circumference to the height of the billet cross section reaches 8:1. Based on this, it is then extruded along the length of the billet to make the thickness, width and length ratio of the formed billet 1.3:1:1.4.

[0052] After annealing, the billet is rolled in eight passes along the thickness direction at the initial rolling temperature, alternating between the length and width directions for one hot rolling pass. After each pass, the billet is rotated 90° until the ratio of initial rolling thickness to final rolling thickness is 260.0%. The initial pass reduction rate is controlled at 11%, the intermediate pass reduction rate at 16%, and the final rolling pass reduction rate at 18%.

[0053] The billet is heated and then subjected to secondary hot rolling. In this secondary hot rolling, the billet is rolled down in seven passes along the thickness direction, alternating between the length and width directions. After each pass, the billet is rotated 90° until the ratio of the initial rolling thickness to the final rolling thickness is 280.0%. The initial pass reduction rate is controlled at 5%, the intermediate pass reduction rate at 14%, and the final rolling pass reduction rate at 14%.

[0054] Example 2 Vacuum induction smelting ingot, the ingot comprising the following components by mass percentage: C: 0.32%, Mo: 0.4%, Ni: 4.5%, Si: 2.5%, Mn: 0.8%, Cr: 0.9%, V: 0.7%, Nb: 0.010%, S: 14ppm, O: 12ppm, N: 8ppm, with the balance being Fe and unavoidable impurities, wherein Ni:Cr = 5:1, V:Nb = 70:1.

[0055] After peeling and cutting off the top and bottom of the ingot, it is smelted in vacuum to become a vacuum consumable ingot.

[0056] The vacuum consumable ingot was heated to 1150℃ and held at that temperature to allow the Nb-V composite primary carbide to fully dissolve. The ingot diameter was 300mm, Mo+Nb≤1.2%, k was taken as 6, and the holding time was 1800min.

[0057] The vacuum consumable ingot is extruded along its length until the ratio of the circumference to the height of the billet cross section reaches 9:1. Based on this, it is then extruded along the length of the billet to make the thickness, width and length ratio of the formed billet 1:1:1.4.

[0058] After annealing, the billet is rolled in seven passes along the thickness direction at the initial rolling temperature, alternating between the length and width directions for hot rolling. After each pass, the billet is rotated 90° until the ratio of initial rolling thickness to final rolling thickness is 275.0%. The initial pass reduction rate is controlled at 12%, the intermediate pass reduction rate at 20%, and the final rolling pass reduction rate at 20%.

[0059] The billet is heated and then subjected to secondary hot rolling. In this secondary hot rolling, the billet is rolled down in six passes along the thickness direction, alternating between the length and width directions. After each pass, the billet is rotated 90° until the ratio of the initial rolling thickness to the final rolling thickness is 280.0%. The initial pass reduction rate is controlled at 6%, the intermediate pass reduction rate at 10%, and the final pass reduction rate at 12%.

[0060] Example 3 Vacuum induction smelting ingot, the ingot comprising the following components by mass percentage: C: 0.38%, Mo: 0.8%, Ni: 3.5%, Si: 1.8%, Mn: 0.5%, Cr: 1.75%, V: 0.4%, Nb: 0.020%, S: 6ppm, O: 8ppm, N: 9ppm, with the balance being Fe and unavoidable impurities, wherein Ni:Cr = 2:1, V:Nb = 20:1.

[0061] After peeling and cutting off the top and bottom of the ingot, it is smelted in vacuum to become a vacuum consumable ingot.

[0062] The vacuum consumable ingot was heated to 1160℃ and held to allow the Nb-V composite primary carbide to fully dissolve. The ingot diameter was 280mm, Mo+Nb≤1.2%, k was taken as 6.5, and the holding time was 1820min.

[0063] The vacuum consumable ingot is extruded along its length until the ratio of the circumference to the height of the billet cross section reaches 8.5:1. Based on this, it is then extruded along the length of the billet to make the thickness, width and length ratio of the formed billet 1.5:1:1.1.

[0064] After annealing, the billet is rolled in five passes along the thickness direction at the initial rolling temperature, alternating between the length and width directions for one hot rolling operation. After each pass, the billet is rotated 90° until the ratio of initial rolling thickness to final rolling thickness is 275.0%. The initial pass reduction rate is controlled at 7%, the intermediate pass reduction rate at 15%, and the final rolling pass reduction rate at 20%.

[0065] The billet is heated and then subjected to secondary hot rolling. In this secondary hot rolling, the billet is rolled down in six passes along the thickness direction, alternating between the length and width directions. After each pass, the billet is rotated 90° until the ratio of the initial rolling thickness to the final rolling thickness is 300.0%. The initial pass reduction rate is controlled at 5%, the intermediate pass reduction rate at 13%, and the final pass reduction rate at 12%.

[0066] Comparative Example 1 The smelting and processing operations of Comparative Example 1 and Example 1 are basically the same. The difference between the two lies in the composition. In Comparative Example 1, Ni: 3.0%, Cr: 1.5%, and the other components are the same.

[0067] In step S500, the billet cracks during the rolling process due to the poor plasticity of the material, making it impossible to process and shape it according to the required process.

[0068] Comparative Example 2 The smelting and processing operations of Comparative Example 2 are basically the same as those of Example 1. The difference between the two lies in their composition; Comparative Example 2 does not contain V, while the other components are the same. In this comparative example, due to the absence of V, chain-like NbC is present in the steel, resulting in a decrease in the steel's transverse impact properties.

[0069] Comparative Example 3 The smelting and processing operations of Comparative Example 3 were basically the same as those of Example 1. The difference between the two lies in the composition. In Comparative Example 3, V: 0.17%, Nb: 0.020%, and the remaining components were the same. In this comparative example, due to the low V content, large-sized NbC particles were present in the steel, resulting in a decrease in the transverse and longitudinal impact properties of the steel.

[0070] Comparative Example 4 The composition and smelting operation of Comparative Example 4 are basically the same as those of Example 1. The difference between the two lies in the hot working process. Comparative Example 4 adopts a four-upsetting and four-drawing forging process.

[0071] Comparative Example 5 The composition and operation of Comparative Example 5 are basically the same as those of Example 1. The difference between the two is that Comparative Example 5 did not undergo heating and remelting, but directly underwent extrusion deformation and subsequent operations.

[0072] The mechanical properties of the steels prepared in Examples 1-3 and Comparative Examples 2-5 were tested, and the results are shown in Table 1 below: Table 1 Mechanical properties of the steels obtained in Examples 1-3 and Comparative Examples 2-5

[0073] As shown in Table 1 above, this method effectively reduces the difference in transverse and longitudinal properties of medium and low alloy ultra-high strength steel. The average longitudinal tensile strength is ≥1940MPa, the average longitudinal yield strength is ≥1530MPa, and the average longitudinal U-shaped impact toughness is ≥50J. The ratio of the average transverse and longitudinal tensile strength is above 99%, the ratio of the average transverse and longitudinal yield strength is above 98%, and the ratio of the average transverse and longitudinal U-shaped impact toughness is above 100%.

[0074] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the disclosed embodiments of the present invention is limited to these examples; within the framework of the embodiments of the present invention, the technical features of the above embodiments or different embodiments can also be combined, and there are many other variations of different aspects of the embodiments of the present invention as described above, which are not provided in detail for the sake of brevity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the embodiments of the present invention should be included within the protection scope of the embodiments of the present invention.

Claims

1. A method for mitigating the difference in transverse and longitudinal properties of ultra-high strength steel materials, characterized in that, Includes the following steps: An ingot is produced by vacuum induction smelting. The ingot comprises the following components by mass percentage: C: 0.32%~0.38%, Mo: 0.4%~0.8%, Ni: 3.5%~4.5%, Si: 1.0%~2.5%, Mn: 0.4%~0.8%, Nb: 0.010%~0.020%, Cr and V, S: 5~15ppm, O: 5~15ppm, N: 5~15ppm, with the balance being Fe and unavoidable impurities, wherein Ni:Cr = 2~5:1, V:Nb = 20~70:1; The ingot is smelted into a vacuum consumable ingot through vacuum consumable metallurgy. The vacuum consumable ingot is heated and kept at a constant temperature until the Nb-V composite primary carbide is fully dissolved. The vacuum consumable ingot is extruded along its length until the ratio of the cross-sectional perimeter to the height of the billet reaches a predetermined ratio, and then shaped into a billet suitable for rolling. The billet is pressed down along the thickness direction, and hot rolling is performed once by alternately using the length direction and the width direction, which are perpendicular to the thickness direction, as the rolling direction, until the thickness change of the billet meets the requirements; The billet is heated and then subjected to secondary hot rolling. In the secondary hot rolling, the billet is pressed down along the thickness direction, and the length direction and the width direction are alternately used as the rolling direction. The reduction rate of each pass in the secondary hot rolling is different from that of each pass in the primary hot rolling.

2. The method for mitigating the difference in transverse and longitudinal properties of ultra-high strength steel materials according to claim 1, characterized in that, The vacuum consumable ingot is heated to 1150~1180℃ and kept warm.

3. The method for mitigating the difference in transverse and longitudinal properties of ultra-high strength steel materials according to claim 1, characterized in that, The heat preservation time is calculated based on the diameter of the vacuum consumable ingot and determined by the following formula: t=k*D, Where t is the heat preservation time in min; k is the heat preservation coefficient, which is related to the content of Mo and Nb elements in the material and is in min / mm; and D is the diameter of the vacuum consumable ingot in mm.

4. The method for mitigating the difference in transverse and longitudinal properties of ultra-high strength steel materials according to claim 1, characterized in that, The vacuum consumable ingot is extruded along its length until the ratio of the cross-sectional perimeter to the height of the steel billet reaches 8-9:

1.

5. The method for mitigating the difference in transverse and longitudinal properties of ultra-high strength steel materials according to claim 1, characterized in that, The thickness, width and length ratio of the billet suitable for rolling is (1~1.5):1:(1~1.5).

6. The method for mitigating the difference in transverse and longitudinal properties of ultra-high strength steel materials according to claim 1, characterized in that, The hot rolling process includes 5 to 8 passes, with the ratio of the initial thickness of the billet to the final rolling thickness being 2.2 to 2.8:

1. After each pass, the billet needs to be rotated 90°.

7. The method for mitigating the difference in transverse and longitudinal properties of ultra-high strength steel materials according to claim 6, characterized in that, The reduction rate for each pass of hot rolling is controlled as follows: 7%~12% for the first pass, 14%~21% for the intermediate passes, and 17%~20% for the last pass.

8. The method for mitigating the difference in transverse and longitudinal properties of ultra-high strength steel materials according to claim 1, characterized in that, The secondary hot rolling includes 5 to 7 passes, and the ratio of the initial thickness of the billet to the final rolling thickness is 2.5 to 3.0:

1. After each pass, the billet needs to be rotated 90°.

9. The method for mitigating the difference in transverse and longitudinal properties of ultra-high strength steel materials according to claim 1, characterized in that, The rolling direction of the first pass of the secondary hot rolling is perpendicular to the rolling direction of the last pass of the primary hot rolling.

10. The method for mitigating the difference in transverse and longitudinal properties of ultra-high strength steel materials according to claim 1, characterized in that, The reduction rate for each pass of the secondary hot rolling is controlled as follows: 4%~6% for the first pass, 10%~15% for the intermediate passes, and 12%~14% for the last pass.

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