Medium-low alloy ultrahigh strength steel and preparation method thereof

Abstract

The invention discloses medium-low alloy ultra-high strength steel and a preparation method thereof, the anisotropy index of the medium-low alloy ultra-high strength steel is 88%-100%, and the anisotropy index is equal to transverse performance / longitudinal performance. The preparation method comprises the steps that a consumable remelting ingot is obtained, and according to the mass percentage, the purity requirement is that S is smaller than or equal to 0.001%, P is smaller than or equal to 0.002%, O is smaller than or equal to 0.0015%, N is smaller than or equal to 0.0050%, and Ti is smaller than or equal to 0.0050%; carrying out high-temperature homogenization treatment on the consumable remelting ingot; upsetting and drawing cogging is conducted on the self-consuming remelting ingot subjected to homogenization treatment, the total forging ratio of upsetting and drawing cogging is not smaller than 15, an intermediate billet is obtained, one-time heating forming is conducted on the intermediate billet, and a formed bar with the required specification is obtained; and the formed bar is subjected to normalizing and ultra-refining and then is annealed and put in storage, and the medium-low alloy ultra-high strength steel is obtained. The longitudinal and transverse performance difference of the medium-low alloy ultrahigh-strength steel bar is reduced, and the problem that the transverse and longitudinal toughness and plasticity difference of an existing medium-low alloy ultrahigh-strength steel bar is too large is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of railway steel production, and particularly to a medium-low alloy ultra-high strength steel and a preparation method thereof. Background Art

[0002] In medium-low alloy ultra-high strength steel, the main elements generally mainly include C, Cr, Mn, Si, Ni, Mo, W, etc., and the content of main alloying elements ≤ 10%. Among them, the range of C is 0.16 - 0.45%, the range of Cr is 0.5 - 4.0%, the range of Mn is 0.2 - 1.5%, the range of Si ≤ 2.5%, the range of Ni ≤ 5%, the range of Mo is 0.2 - 1.5%, the range of W ≤ 3.5%, and the elements added in trace amounts are V and Nb, with the range of V being 0.01 - 0.40% and the range of Nb being 0.01 - 0.05%. Typical steel grades include 4340 (40CrNi2MoA), D6AC (45CrNiMo1VA), N31 (30CrMnSiNi2A), D406A (30Si2MnCrMoVE), 300M (40Si2Ni2CrMoVA), etc.

[0003] Metal materials usually have anisotropy, that is, they have different physical and mechanical properties in different directions. During the rolling or forging process, the grains of the metal will extend along the processing direction, resulting in different longitudinal and transverse properties. The longitudinal properties are usually enhanced due to metal flow and grain extension, so the longitudinal properties are generally better than the transverse properties. For example, the elongation and reduction of area differences in longitudinal and transverse tensile tests are large, and there are also large differences in longitudinal and transverse impact properties and fracture toughness.

[0004] When preparing medium-low alloy ultra-high strength steel bars, different requirements are often made for the reduction of area and elongation in longitudinal and transverse directions of tensile properties in the standard, as well as impact properties and fracture toughness. For example, for 30CrMnSiNi2A steel bars with Φ ≤ 200mm in longitudinal and transverse directions, the longitudinal tensile A ≥ 9%, Z ≥ 45%, the transverse tensile A ≥ 5%, Z ≥ 25%; for steel bars with 200 < Φ ≤ 300mm, the longitudinal tensile A ≥ 8%, Z ≥ 40%, the transverse tensile A ≥ 5%, Z ≥ 25%; the tensile strength and yield strength are the same: Rm ≥ 1620 Mpa, Rp0.2 ≥ 1375 Mpa.

[0005] At present, the main melting methods for medium-low alloy ultra-high strength steel are EBT + LF + VD + MC / vacuum induction furnace (VIM) + electroslag remelting (ESR) / vacuum arc consumable remelting (VAR), and the forming is free forging upsetting and cogging, and free forging forming.

[0006] The existing technology pays more attention to improving the longitudinal properties of medium and low alloy ultra-high strength steel bars, but pays insufficient attention to the transverse properties. As a result, the transverse properties of the material may become a limiting factor in some applications. Improving the transverse properties of medium and low alloy ultra-high strength steel bars is a multi-faceted issue that requires comprehensive consideration, and requires systematic technical improvements from multiple angles such as purity, micro-segregation of components, type and size of inclusions, size and distribution of carbides, heat treatment and hot forming.

[0007] Based on this, the existing technology still needs to be improved. Summary of the invention

[0008] In order to solve the above technical problems, an embodiment of the present invention provides a medium-low alloy ultra-high strength steel and a preparation method thereof.

[0009] To solve the above technical problems, on the one hand, some embodiments of the present invention disclose a medium-low alloy ultra-high strength steel with an anisotropy index of 88%-100%, wherein the anisotropy index = transverse performance / longitudinal performance.

[0010] In some embodiments, the above-mentioned medium and low alloy ultra-high strength steel includes main elements and trace additive elements. Calculated by mass percentage, the content of the main elements is ≤10%, and the main elements include: C 0.16-0.45%, Cr0.5-4.0%, Mn0.2-1.5%, Si≤2.5%, Ni≤5%, Mo0.2-1.5%, W≤3.5%; the trace additive elements include V and Nb.

[0011] In some embodiments, by mass percentage, V is 0.01-0.40%, and Nb is 0.01-0.05%.

[0012] On the other hand, the embodiment of the present invention also discloses a method for preparing the aforementioned medium-low alloy ultra-high strength steel, comprising: Step 1: Obtain a consumable remelting ingot, and the purity requirements are as follows: S≤0.001%, P≤0.002%, O≤0.0015%, N≤0.0050%, Ti≤0.0050% in terms of mass percentage; Step 2: performing high temperature homogenization treatment on the consumable remelting ingot; Step 3: The consumable remelted ingot after homogenization is subjected to upsetting and drawing to form a blank, and the total forging ratio of the upsetting and drawing blank is ≥15 to obtain an intermediate blank, and the intermediate blank is formed by one-time fire to obtain a formed bar of required specifications; Step 4: After normalizing and ultra-refining the formed bars, annealing and storage are carried out to obtain medium-low alloy ultra-high strength steel.

[0013] In some embodiments, in step one, on the premise that S≤0.001%, 0.001-0.005wt% Ca or 0.005-0.01wt% RE is added at the end of the first melting to modify the MnS inclusions in the steel.

[0014] In some embodiments, in step two, the high-temperature homogenization treatment is as follows: treatment at a temperature of 1200°C ± 20°C for 20h - 40h.

[0015] In some embodiments, in step three, for the upsetting and drawing to open the billet, three-upsetting and three-drawing or four-upsetting and four-drawing are adopted.

[0016] In some embodiments, in step three, the heating temperature for one-fire forming is 1060°C ± 10°C, and the forging reduction is ≥50%.

[0017] In some embodiments, in step four, the number of normalizing and ultra-refining is 2 - 4 times, and the normalizing process is air cooling for 60 - 70 min at 920°C ± 10°C.

[0018] In some embodiments, in step four, the annealing process is heat preservation for 120 - 130 min at 680°C ± 10°C.

[0019] Adopting the above technical solutions, the present invention has at least the following beneficial effects: A medium-low alloy ultra-high strength steel and a preparation method thereof provided by the present invention, through the preparation process of the medium-low alloy ultra-high strength steel bar, improve the purity, modify the inclusions, make the composition more uniform, the structure and the precipitated phases more uniform, reduce the longitudinal and transverse property differences of the medium-low alloy ultra-high strength steel bar, and solve the problem of too large difference in transverse and longitudinal toughness and plasticity of the existing medium-low alloy ultra-high strength steel bar. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0021] Figure 1 It is a flowchart of a preparation method of a medium-low alloy ultra-high strength steel disclosed in some embodiments of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] The following further describes the embodiments of the present disclosure in conjunction with the accompanying drawings and embodiments. The detailed descriptions and drawings of the following embodiments are used to exemplarily illustrate the principles of the present disclosure, but cannot be used to limit the scope of the present disclosure. The present disclosure can be implemented in many different forms, not limited to the specific embodiments disclosed herein, but including all technical solutions falling within the scope of the claims.

[0023] The present disclosure provides these embodiments to make the present disclosure thorough and complete, and to fully convey the scope of the present disclosure to those skilled in the art. It should be noted that: unless otherwise specifically stated, the relative arrangements of components and steps, the components of materials, numerical expressions and values set forth in these embodiments should be construed as merely exemplary, rather than as limitations.

[0024] It should be noted that in the description of the present disclosure, unless otherwise specified, the meaning of "a plurality" is greater than or equal to two; the orientation or positional relationships indicated by terms such as "upper", "lower", "left", "right", "inner", "outer", etc. are only for the convenience of describing the present disclosure and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus cannot be construed as a limitation of the present disclosure. When the absolute position of the object being described changes, the relative positional relationship may also change accordingly.

[0025] In addition, the "first", "second" and similar terms used in the present disclosure do not denote any order, quantity or importance, but are only used to distinguish different parts. "Vertical" is not strictly vertical, but within the allowable error range. "Parallel" is not strictly parallel, but within the allowable error range. Terms such as "including" or "comprising" mean that the elements before the term cover the elements listed after the term, and do not exclude the possibility of also covering other elements.

[0026] It should also be noted that in the description of the present disclosure, unless otherwise clearly specified and limited, the terms "mounted", "connected" and "connected" should be understood in a broad sense. For example, it may be a fixed connection, a detachable connection, or an integral connection; it may be directly connected, or indirectly connected through an intermediate medium. For those of ordinary skill in the art, the specific meanings of the above terms in the present disclosure can be understood according to specific circumstances. When it is described that a specific device is located between a first device and a second device, there may or may not be an intermediate device between the specific device and the first device or the second device.

[0027] All terms used in this disclosure have the same meanings as those understood by those of ordinary skill in the art to which this disclosure pertains, unless otherwise specifically defined. It should also be understood that terms defined in a general dictionary, such as those, should be interpreted as having meanings consistent with their meanings in the context of the relevant art, and should not be interpreted in an idealized or overly formal sense, unless specifically defined as such herein.

[0028] Technologies, methods, and devices known to those of ordinary skill in the relevant art may not be discussed in detail, but where appropriate, the technologies, methods, and devices should be regarded as part of the specification.

[0029] As Figure 1 shown, embodiments of the present invention disclose a medium-low alloy ultra-high strength steel and a preparation method thereof. The longitudinal and transverse properties of the medium-low alloy ultra-high strength steel have small differences, and its anisotropy index is 88% - 99.5%. Among them, the anisotropy index = transverse property / longitudinal property. That is, the anisotropy index AR 性能 (Anisotropy Ratio) = transverse property / longitudinal property, AR Rm = transverse Rm / longitudinal Rm. For the medium-low alloy ultra-high strength steel bars with a specification of Φ200 - Φ400mm prepared by this method, AR Rm ≥ 99.5%, AR Rp0.2 ≥ 99.5%, AR A5 ≥ 90%, AR Z ≥ 90%, AR aku2 ≥ 88%. It solves the problem of the excessive gap between the transverse and longitudinal toughness and plasticity of existing medium-low alloy ultra-high strength steel bars.

[0030] Its preparation method mainly includes: 1) Improve its purity through the EBT + LF + VD + MC / vacuum induction furnace (VIM) + electroslag remelting (ESR) / vacuum arc consumable remelting (VAR) melting process of medium-low alloy ultra-high strength steel. The specific requirements are S ≤ 0.001%, P ≤ 0.002%, O ≤ 0.0015%, N ≤ 0.0050%, Ti ≤ 0.0050%.

[0031] 2) When the medium-low alloy ultra-high strength steel undergoes electroslag remelting (ESR) / vacuum arc consumable remelting (VAR), low melting speed and shallow flat molten pool melting are carried out to increase the local solidification time of the remelted solidification structure, reduce the secondary dendrite arm spacing, and reduce the compositional segregation between dendrites. The remelted ingot of the medium-low alloy ultra-high strength steel is subjected to high-temperature homogenization at 1200°C ± 20°C * 30h ± 10h before upsetting and drawing to further reduce the compositional micro-segregation between dendrites of the solidification structure.

[0032] 3) Forging and cogging adopt multi-pass upsetting and drawing, and the total forging ratio of upsetting and drawing cogging is ≥15. After forging and cogging into an intermediate billet, the intermediate billet is formed into the final bar specification in one heat, the heating temperature is 1060°C ± 10°C, and the forging reduction is ≥50%.

[0033] 4) The formed bars are normalized and ultra-fined 2 - 4 times and then annealed and stored. The normalizing process is air cooling at 920°C ± 10°C * 60 min.

[0034] In the above step 1), the medium-low alloy ultra-high strength steel means that the main elements mainly include C, Cr, Mn, Si, Ni, Mo, W, etc., and the content of the main alloying elements is ≤10%. Among them, the range of C is 0.16 - 0.45%, the range of Cr is 0.5 - 4.0%, the range of Mn is 0.2 - 1.5%, the range of Si is ≤2.5%, the range of Ni is ≤5%, the range of Mo is 0.2 - 1.5%, the range of W is ≤3.5%, and the trace elements added are V and Nb, the range of V is 0.01 - 0.40% and the range of Nb is 0.01 - 0.05%.

[0035] In the above step 1), as a preferred implementation, on the premise of requiring S ≤ 0.001%, a small amount of Ca or RE can be added at the end of the first melting to modify the MnS inclusions in the steel, so that the long strip-shaped MnS is transformed into spherical CaS or rare earth sulfide.

[0036] In the above step 3), as a preferred implementation, it can be upset 3 times and drawn 3 times, or upset 4 times and drawn 4 times.

[0037] In the above step 3), as a preferred implementation, the size of the intermediate billet for forging and cogging can be designed according to the final formed specification, so that the forging reduction during the last heat forming is ≥50%.

[0038] In the above step 4), as a preferred implementation, the normalizing and ultra-fining process can be carried out 3 or 4 times, and the organization is made more uniform by multiple phase transformations.

[0039] In the prior art, little consideration is given to the gap between the transverse properties and longitudinal properties of medium-low alloy ultra-high strength steel bars, and it is basically considered as an inherent characteristic of the bars. Through the preparation process of medium-low alloy ultra-high strength steel bars in the above embodiments of the present invention, the purity is improved, the inclusions are modified, the composition is more uniform, the structure and precipitated phases are more uniform, and the longitudinal and transverse property differences of medium-low alloy ultra-high strength steel bars are reduced. Specifically: 1) Improve purity and modify inclusions: Reduce the content of residual elements in steel, especially reduce the content of segregation-prone elements S and P. Reduce and modify the deformable inclusions in steel, such as MnS. Since Mn is an alloying element in medium and low alloy ultra-high strength steel, and the Mn content ranges from 0.4% to 1.3%, it is necessary to strictly control the S content in steel. Ca and RE can also be added to modify MnS inclusions. MnS inclusions are prone to extend along the deformation direction, forming large-sized strip inclusions, which may lead to deterioration of the transverse properties of steel.

[0040] 2) Composition uniformity: By consumable remelting (ESR or VAR) with low melting rate and shallow flat molten pool smelting, the local cooling rate at each position of the ingot can be increased, which can refine the dendritic structure and reduce the solute segregation between dendrites. Then, through high-temperature homogenization, the microscopic segregation of composition between dendrites in the solidification structure is improved, making the composition of the ingot more uniform in all directions, thereby improving the transverse properties of steel.

[0041] 3) Microstructure uniformity: Through multiple upsetting and drawing operations, the total forging ratio of upsetting and drawing billet opening is ≥15. After forging and billet opening into an intermediate billet, the final bar specification is formed in one heat using the intermediate billet, with a heating temperature of 1060°C ± 10°C and a forging reduction of ≥40%. Improve the anisotropy of the microstructure of the bar.

[0042] 4) Microstructure and precipitation phase uniformity: Through the ultra-fine grain process of 2 - 4 cycles of normalizing, the grain orientation and precipitation phase distribution in the longitudinal and transverse directions of the bar are made relatively more uniform.

[0043] The present invention will be further described below in conjunction with embodiments, but the present invention is not limited to the following embodiments: Example 1 A method for reducing the longitudinal and transverse property differences of medium and low alloy ultra-high strength steel bars. Taking the preparation of Φ230mm and Φ350mm specification bars from medium and low alloy ultra-high strength steel 300M (40Si2Ni2CrMoVA) as an example, the specific operation steps are as follows: (1) Use the method of vacuum induction melting (VIM) + vacuum arc consumable remelting (VAR) to smelt a Φ660mm consumable remelted ingot of 300M steel, with a high purity control level. The specific residual elements detected are: S 0.0007%, P 0.0016%, O 0.0010%, N 0.0010%, Ti 0.0025%.

[0044] (2) The Φ660mm consumable remelted ingot of 300M steel is subjected to high-temperature homogenization at 1200°C for 30h before upsetting and drawing billet opening to further reduce the microscopic segregation of composition between dendrites in the solidification structure.

[0045] (3) Forging and cogging adopt 3 upsetting and 3 drawing operations, and the total forging ratio of upsetting and drawing is 16. The total forging ratio of upsetting and drawing refers to the sum of the forging ratios in each process of three upsetting and three drawing operations. The forging ratio of the first upsetting is 3, the forging ratio of the first drawing is 3, the forging ratio of the second upsetting is 3, the forging ratio of the second drawing is 3, the forging ratio of the third upsetting is 2, and the forging ratio of the third drawing is 2. The sum of the total forging ratios is 3 + 3 + 3 + 3 + 2 + 2 = 16. After forging and cogging into an intermediate billet of Φ500mm, the intermediate billet is formed in one heat. The heating temperature is 1070°C, and the final formed bars are of Φ230mm and Φ350mm specifications. The forging reduction ratios are 78.8% and 51% respectively.

[0046] (4) The formed bars of Φ230mm and Φ350mm specifications are normalized three times for super-refinement and then annealed and stored in the warehouse. The normalizing process is 920°C × 60min in air, and the annealing process is: 680°C × 120min, air cooling.

[0047] (5) Using the standard heat treatment system of 300M steel (870°C × 1h oil quenching + 300°C × 2h air cooling + 300°C × 2h air cooling), the longitudinal and transverse tensile and U-notch impact properties of the 1 / 2R area of the bars of Φ230mm and Φ350mm specifications are detected, and the anisotropy index AR 性能 (Anisotropy Ratio) = transverse property / longitudinal property. The results are shown in Table 1 and Table 2.

[0048] Example 2 A method for reducing the longitudinal and transverse property differences of medium and low alloy ultra-high strength steel bars. Taking the preparation of bars of Φ230mm and Φ350mm specifications from medium and low alloy ultra-high strength steel 300M (40Si2Ni2CrMoVA) as an example, the specific operation steps are as follows: (1) The 300M steel Φ660mm consumable remelted ingot is smelted by the method of EBT + LF + VD + MC + vacuum arc consumable (VAR), and the purity control level is high. The specific residual elements are: S 0.0008%, P 0.0016%, O 0.0011%, N 0.0018%, Ti 0.0028%. Before the LF ladle is lifted, 0.3kg / t of SiCa wire is fed to further desulfurize and modify the MnS inclusions in the steel.

[0049] (2) The 300M steel Φ660mm consumable remelted ingot is subjected to high-temperature homogenization at 1200°C * 30h before upsetting and cogging to further reduce the compositional micro-segregation between dendrites in the solidification structure.

[0050] (3)Forging and cogging adopt three upsetting and three drawing operations, and the sum of the total deformation ratios of upsetting and drawing is 16. The total forging ratio of upsetting and drawing refers to the sum of the forging ratios in each process of three upsetting and three drawing operations. The forging ratio of the first upsetting is 3, the forging ratio of the first drawing is 3, the forging ratio of the second upsetting is 3, the forging ratio of the second drawing is 3, the forging ratio of the third upsetting is 2, and the forging ratio of the third drawing is 2. The sum of the total forging ratios is 3 + 3 + 3 + 3 + 2 + 2 = 16. After forging and cogging into an intermediate billet with a diameter of Φ500mm, the intermediate billet is formed in one heat. The heating temperature is 1070°C, and the final formed bars are of Φ230mm and Φ350mm specifications. The forging reduction ratios are 78.8% and 51.0% respectively.

[0051] (4)Furthermore: After the formed bars of Φ230mm and Φ350mm specifications are super-refined by normalizing three times, they are annealed and stored in the warehouse. The normalizing process is 920°C × 60min in air, and the annealing process is: 680°C × 120min, air cooling.

[0052] (5)Using the standard heat treatment system of 300M steel (870°C × 1h oil quenching + 300°C × 2h air cooling + 300°C × 2h air cooling), the longitudinal and transverse tensile and U-notch impact properties of the 1 / 2R area of the bars of Φ230mm and Φ350mm specifications are detected, and the anisotropy index AR 性能 (Anisotropy Ratio) = transverse property / longitudinal property. The results are shown in Table 1 and Table 2.

[0053] Example 3 A method for reducing the longitudinal and transverse property differences of medium-low alloy ultra-high strength steel bars. Taking the preparation of bars of Φ230mm and Φ350mm specifications from medium-low alloy ultra-high strength steel 300M (40Si2Ni2CrMoVA) as an example, the specific operation steps are as follows: (1)Using the method of EBT + LF + VD + MC + electroslag remelting (ESR) to smelt and obtain a 300M steel Φ770mm consumable remelted ingot with a high purity control level. The specific residual elements are: S 0.007, P 0.0016%, O 0.0018%, N 0.0042%, Ti 0.0015%.

[0054] (2)The 300M steel Φ770mm consumable remelted ingot is subjected to high-temperature homogenization at 1200°C × 36h before upsetting and cogging to further reduce the compositional micro-segregation between dendrites in the solidification structure.

[0055] (3) Forging and cogging uses 4 upsetting and 4 drawing processes, and the sum of the total deformation ratios of upsetting and drawing is 20. The total forging ratio of upsetting and drawing refers to the sum of the forging ratios in each of the four upsetting and four drawing processes. The forging ratio for the first upsetting is 3, the forging ratio for the first drawing is 3, the forging ratio for the second upsetting is 3, the forging ratio for the second drawing is 3, the forging ratio for the third upsetting is 2, the forging ratio for the third drawing is 2, the forging ratio for the fourth upsetting is 2, and the forging ratio for the fourth drawing is 2. The sum of the total forging ratios is 3 + 3 + 3 + 3 + 2 + 2 + 2 + 2 = 20. After forging and cogging into an intermediate billet of Φ550mm, the intermediate billet is formed in one heat. The heating temperature is 1070°C, and the final formed bars are of Φ230mm and Φ350mm specifications. The forging reduction ratios are 82.5% and 59.5% respectively.

[0056] (4) Further: After the formed bars of Φ230mm and Φ350mm specifications are super-refined by normalizing 3 times, they are annealed and stored in the warehouse. The normalizing process is 920°C × 60min in air, and the annealing process is: 680°C × 120min, air cooling.

[0057] (5) Using the standard heat treatment system of 300M steel (870°C × 1h oil quenching + 300°C × 2h air cooling + 300°C × 2h air cooling), the longitudinal and transverse tensile and U-notch impact properties of the 1 / 2R area of the bars of Φ230mm and Φ350mm specifications are detected, and the anisotropy index AR 性能 (Anisotropy Ratio) = transverse property / longitudinal property. The results are shown in Table 1 and Table 2.

[0058] Comparative Example 1 Taking the preparation of bars of Φ230mm and Φ350mm specifications from medium-low alloy ultra-high strength steel 300M (40Si2Ni2CrMoVA) as a comparative example, the specific operation steps are as follows: (1) Using the method of vacuum induction (VIM) + vacuum arc consumable (VAR) to smelt and obtain a 300M steel Φ660mm consumable remelted ingot, with a relatively high purity control level. The specific residual elements are: S 0.00010%, P 0.0020%, O 0.0010%, N 0.0010%, Ti 0.0025%.

[0059] (2) Forging and cogging uses 2 upsetting and 2 drawing processes, and the sum of the total deformation ratios of upsetting and drawing is 10 and 12. Forging and cogging into intermediate billets of Φ450mm (the sum of the total deformation ratios of upsetting and drawing is 10) and Φ350mm (the sum of the total deformation ratios of upsetting and drawing is 12). The heating temperature is 1150°C. Using the Φ450mm intermediate billet to form the Φ350mm specification bar in one heat, the forging reduction ratio is 39.5%. The heating temperature is 1150°C. Using the Φ350mm intermediate billet to form the Φ230mm specification bar in one heat, the forging reduction ratio is 56.8%.

[0060] (3) Further, the formed bars with specifications of Φ230mm and Φ350mm are stored in the warehouse after normalizing + annealing. The normalizing process is 920°C × 60min in air, and the annealing process is 680°C × 120min, air cooling.

[0061] (4) Using the standard heat treatment system of 300M steel (870°C × 1h oil quenching + 300°C × 2h air cooling + 300°C × 2h air cooling), the longitudinal and transverse tensile and U-notch impact properties of the 1 / 2R area of the bars with specifications of Φ230mm and Φ350mm are detected, and the anisotropy index AR 性能 (Anisotropy Ratio) = transverse property / longitudinal property. The results are shown in Table 1 and Table 2.

[0062] Table 1 Comparison of anisotropy index of Φ230mm bars in Examples 1 - 3 and Comparative Example 1

[0063] Table 2 Comparison of anisotropy index of Φ350mm bars in Examples 1 - 3 and Comparative Example 1

[0064] The above has introduced in detail a method for reducing the longitudinal and transverse property differences of medium - low alloy ultra - high strength steel bars provided by the present invention. Specific examples are used in this article to elaborate on the principle and implementation mode of the present invention. The description of the above examples is only used to help understand the method and its core idea of the present invention. It should be noted that for those of ordinary skill in the art in this technical field, without departing from the principle of the present invention, several improvements and modifications can be made to the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

[0065] So far, the embodiments of the present disclosure have been described in detail. To avoid obscuring the concept of the present disclosure, some details well - known in the art have not been described. Those skilled in the art can fully understand how to implement the technical solutions disclosed here based on the above description.

[0066] Although some specific embodiments of the present disclosure have been described in detail by way of examples, those skilled in the art should understand that the above examples are only for illustration and not for limiting the scope of the present disclosure. Those skilled in the art should understand that the above embodiments can be modified or some technical features can be equivalently replaced without departing from the scope and spirit of the present disclosure. In particular, as long as there is no structural conflict, the technical features mentioned in each embodiment can be combined in any way.

Claims

1. A medium and low alloy ultra-high strength steel, characterized in that: The anisotropy index of the medium and low alloy ultra-high strength steel is 88%-99.5%, wherein the anisotropy index = transverse performance / longitudinal performance.

2. The medium-low alloy ultra-high strength steel according to claim 1, characterized in that: It includes main elements and trace additive elements, and the content of the main elements is ≤10% by mass percentage, and the main elements include: C 0.16-0.45%, Cr0.5-4.0%, Mn0.2-1.5%, Si≤2.5%, Ni≤5%, Mo0.2-1.5%, W≤3.5%; the trace additive elements include V and Nb.

3. The medium and low alloy super-strong steel according to claim 1, characterized in that: In terms of mass percentage, V0.01-0.40%, Nb0.01-0.05%.

4. The method for preparing medium-low alloy ultra-high strength steel according to any one of claims 1 to 3, characterized in that: include: Step 1: Obtain a consumable remelting ingot, and the purity requirements are as follows: S≤0.001%, P≤0.002%, O≤0.0015%, N≤0.0050%, Ti≤0.0050% in terms of mass percentage; Step 2: performing high temperature homogenization treatment on the consumable remelting ingot; Step 3: The consumable remelted ingot after homogenization is subjected to upsetting and drawing to form a blank, wherein the total forging ratio of the upsetting and drawing blank is not less than 15, to obtain an intermediate blank, and the intermediate blank is formed by one-time fire to obtain a formed bar of required specifications; Step 4: After normalizing and ultra-refining the formed bars, annealing and storage are carried out to obtain medium-low alloy ultra-high strength steel.

5. The preparation method according to claim 4, characterized in that: In step 1, under the premise of requiring S ≤ 0.001%, 0.001-0.005wt% Ca or 0.005-0.01wt% RE is added at the end of the first smelting to modify the MnS inclusions in the steel.

6. The preparation method according to claim 4, characterized in that: In step 2, the high temperature homogenization treatment is: treating at a temperature of 1200°C±20°C for 20h-40h.

7. The preparation method according to claim 4, characterized in that: In step three, the upsetting and drawing process is performed by three upsetting and three drawing or four upsetting and four drawing.

8. The preparation method according to claim 4, characterized in that: In step three, the heating temperature of the first fire forming is 1060°C±10°C, and the forging reduction is ≥50%.

9. The preparation method according to claim 4, characterized in that: In step 4, the normalizing and ultra-refining times are 2-4 times, and the normalizing process is air cooling at 920°C±10°C for 60-70 minutes.

10. The preparation method according to claim 4, characterized in that: In step 4, the annealing process is to keep the temperature at 680°C ± 10°C for 120-130 minutes.

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