Blade steel and preparation method thereof

Abstract

The invention relates to the field of special steel and discloses blade steel and a preparation method thereof. The method comprises the following steps: smelting an alloy raw material by adopting vacuum magnetic induction, and casting molten steel into an electrode bar; the qualified electrode bar is subjected to electroslag remelting and directionally solidified into a casting blank in a crystallizer, and the casting blank is cooled; heating the casting blank to a second temperature lower than the NbC dissolving temperature by adopting a segmented heating system, and carrying out multi-pass forging; after forging is completed, the forge piece is subjected to controlled cooling; the content of C in the blade steel is 0.05%-0.1%; and the content of Nb is 0.08%-0.1%. According to the method, through optimization of blade steel components, optimization of a smelting process and casting blank quality control, and design of an adaptive forging process and after-forging regulation and control, a forming path of thick and large NbC is blocked, so that the NbC is distributed in a dispersed manner, and the mechanical property and the corrosion resistance of the blade steel are improved.

Description

Technical Field

[0001] This invention relates to the field of special steels, specifically to a blade steel and its preparation method. Background Technology

[0002] Ultra-supercritical power generation technology plays an increasingly important role in promoting the transformation of the national energy structure and ensuring a safe and stable power supply, and has become a key support for the national energy security strategy. As the core equipment of ultra-supercritical generator units, the performance of the steam turbine directly determines the power generation efficiency and environmental protection level, while the blades, as its key core components, have a decisive impact on the reliability, economy and service life of the entire unit, and are also one of the main bottlenecks restricting the development of high-efficiency power generation technology.

[0003] The operating conditions of blade steel for ultra-supercritical generator units are extremely harsh. They operate for extended periods in corrosive steam environments with temperatures exceeding 600°C and supercritical pressures, while simultaneously enduring enormous centrifugal stress, complex bending and torsional stresses, and dynamic vibration loads. This complex stress-corrosion environment, characterized by the superposition of high temperature, high pressure, high stress, and corrosive media, places extremely high demands on the comprehensive performance of the materials. Compared to ordinary blade steel operating only under conventional temperature and pressure conditions, ultra-supercritical blade steel requires higher microstructure homogeneity to ensure isotropy and high-temperature corrosion resistance under these severe conditions.

[0004] However, abnormal microstructures frequently appear in current high-end blade steels, with the core characteristic being the formation of coarse, elongated NbC precipitates. This abnormal microstructure causes the following problems: a sharp drop in substrate hardness, with the hardness of the abnormal microstructure region being only 192-200 HV, far lower than the 328-329 HV of the normal matrix microstructure, failing to meet the mechanical performance design requirements of the blade; the coarse NbC, as a heterogeneous phase, produces significant reflection signals during ultrasonic testing, leading to the workpiece being judged as having defects; due to the strong chemical stability and high corrosion resistance of the NbC phase, while the substrate region experiences a significant decrease in corrosion resistance due to the loss of C and Nb elements, creating a large difference in corrosion resistance compared to the normal matrix microstructure, it is highly susceptible to inducing localized corrosion, seriously affecting the long-term safe service of the blade under complex operating conditions.

[0005] Therefore, there is an urgent need to study a high-end blade steel manufacturing process that can effectively suppress the formation of coarse and elongated NbC precipitates, in order to solve the problems of poor microstructure uniformity, large performance fluctuations, low flaw detection pass rate, and uneven corrosion resistance in existing blade steels. Summary of the Invention

[0006] In view of the above-mentioned problems in the prior art, the main objective of the present invention is to provide a blade steel and a method for preparing the same.

[0007] According to one aspect of the present invention, a method for preparing blade steel is provided, the method comprising the following steps: The alloy raw materials are melted by vacuum magnetic induction. After the melting is qualified, the molten steel is cast into electrode rods in a vacuum environment. After the electrode rods are formed, ultrasonic flaw detection is performed, and the qualified electrode rods are selected. Qualified electrode rods are placed in an electroslag remelting furnace as consumable electrodes for electroslag remelting. The temperature of the molten pool and the remelting speed are controlled. The molten pool is directionally solidified into a billet in the crystallizer. When the billet temperature drops below the first temperature, the billet is removed from the crystallizer and then slowly cooled to room temperature at a rate not exceeding 20°C / min. The billet is heated to a second temperature below the NbC melting temperature using a segmented heating system, and then forged in multiple passes. The deformation amount in the first pass is 15% to 20%, the deformation amount in each of the intermediate passes is 12% to 18%, and the deformation amount in the final forging pass is more than 20% and the total forging deformation amount is more than 70%. After forging, the forging is cooled in a controlled manner. First, it is cooled at a high cooling rate to 650~700℃, then at a lower cooling rate to 250~300℃, and then air-cooled to room temperature. The blade steel comprises the following components by mass fraction: C: 0.05%~0.1%; Nb: 0.08%~0.1%; Cr: 13.8%~15.0%; Ni: 5.5%~7.0%; Mo: 1.3%~1.6%; Cu: 1.9%~2.4%; with the balance being Fe and unavoidable impurities.

[0008] According to one embodiment of the present invention, the alloy raw material includes FeNb alloy, wherein the FeNb alloy has a purity ≥99.8%, Si ≤0.01%, and Al ≤0.005%.

[0009] According to one embodiment of the present invention, the temperature of the vacuum magnetic induction melting is 1580~1620℃. During the melting process, the electromagnetic stirring device is turned on, the stirring frequency is 3~5Hz, and argon gas with a purity ≥99.999% is introduced for bottom blowing, the argon gas flow rate is 0.8~1.2L / min, and the stirring is continued for 40~60min.

[0010] According to one embodiment of the present invention, the casting temperature is 1530~1550°C, and the casting process is cooled at a rate of less than 15°C / min.

[0011] According to one embodiment of the present invention, the slag system used in the electroslag remelting is a CaF2-CaO-Al2O3 ternary slag system, wherein the mass fraction of Al2O3 is 5%~10%, the mass fraction of CaO is 15%~20%, and the balance is CaF2.

[0012] According to one embodiment of the present invention, the parameters of the electroslag remelting are set as follows: After arc ignition, adjust the open-circuit voltage to 65~75V, the working current to 3500~4500A, and maintain the molten pool temperature at 1600~1650℃. Maintain a vacuum level of ≤20Pa inside the furnace during the remelting process.

[0013] According to one embodiment of the present invention, the remelting speed is controlled at 0.4~0.8m / min, the inlet water temperature of the crystallizer is 25~30℃, the outlet water temperature is ≤45℃, and the first temperature is 550~600℃.

[0014] According to one embodiment of the present invention, the segmented heating process includes: heating from room temperature to 600-700°C at a heating rate of 80-100°C / h, holding at that temperature for 1-1.5h; and then heating to 1150-1200°C at a heating rate of 50-70°C / h, holding at that temperature for 2.5-3.5h. The intermediate passes are forged in 2 to 4 passes at a forging temperature of 980 to 1080°C. The forging temperature of the final forging pass is controlled at 890~960℃.

[0015] According to one embodiment of the present invention, the controlled cooling includes: after forging is completed, the forging is immediately moved into a controlled cooling device, first cooled to 650-700°C at a cooling rate of 30-50°C / min, then cooled to 250-300°C at a cooling rate of 10-20°C / min, and finally air-cooled to room temperature.

[0016] According to one embodiment of the present invention, the method further includes annealing the forging after controlled cooling, the annealing process comprising: heating the forging to 550~600°C, holding it at that temperature for 1.5~2.5h, cooling it in the furnace to 300°C, and then air cooling it to room temperature.

[0017] According to another aspect of the present invention, a blade steel is provided, which is prepared by the method according to any of the above embodiments, wherein the blade steel contains NbC with a size not exceeding 4 μm and in a dispersed distribution, has a hardness of 328~336HV, a tensile strength ≥900MPa, a yield strength ≥730MPa, and a retained austenite volume fraction of less than 1%.

[0018] Compared with the prior art, the blade steel and its preparation method of the present invention have at least one of the following beneficial effects: by optimizing the composition of the blade steel, especially the content range of C and Nb, the product of the solid solution concentrations of C and Nb in the steel is controlled within the critical range that can form sufficient NbC to achieve strengthening without causing NbC coarsening, thereby inhibiting the precipitation of coarse NbC; during vacuum magnetic induction smelting, an appropriate smelting temperature is adopted, combined with electromagnetic stirring and argon bottom blowing, to avoid the loss of Nb elements and ensure the uniformity of the steel liquid composition; During the electroslag remelting and refining process, by controlling the molten pool temperature, remelting speed, slag composition, and cooling method, local segregation of Nb was suppressed and the internal stress of the billet was reduced, thus improving the quality of the billet. By optimizing the forging process (including heating, deformation, and cooling) and controlling post-forging cooling, a "multi-pass, small deformation" forging strategy was adopted to break down and refine NbC, while introducing high-density dislocations to inhibit NbC growth. During post-forging cooling, the cooling was controlled to quickly pass through the NbC precipitation-sensitive zone, reducing the chance of NbC precipitation. The blade steel prepared using this method exhibits excellent mechanical and corrosion resistance properties, with a hardness of 328~336 HV, tensile strength ≥900 MPa, yield strength ≥730 MPa, and a retained austenite volume fraction of less than 1%. In a neutral salt spray test (5% NaCl solution, 35℃), the corrosion rate is ≤0.01 mm / year, meeting the long-term safe service requirements of ultra-supercritical generator blades under high temperature, high pressure, high stress, and corrosive environments. Moreover, the NbC size in the blade steel does not exceed 4μm and is diffusely distributed, effectively avoiding problems such as uneven hardness, unqualified flaw detection, and localized corrosion caused by coarse and long strip-shaped NbC precipitates. Attached Figure Description

[0019] 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.

[0020] Figure 1 A flowchart illustrating a method for preparing blade steel according to an embodiment of the present invention is shown; Figure 2 SEM images of blade steel prepared according to Example 1 of the present invention are shown; Figure 3 The images of the blade steel prepared according to Comparative Example 1 are shown, where Figure (a) is a SEM image at 100x magnification, Figure (b) is a SEM image at 5000x magnification, Figure (c) is a surface scan of Nb, and Figure (d) is an EBSD image of the matrix. Detailed Implementation

[0021] 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.

[0022] 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.

[0023] 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.

[0024] Traditional martensitic blade steel, thanks to the synergistic effect of alloying elements such as Cr (13.886%~14.9603%), Ni (5.495%~7.0219%), Mo (1.299%~1.5655%), and Cu (1.8808%~2.398%), possesses excellent corrosion resistance and mechanical properties. However, this blade steel is prone to the formation of coarse, elongated NbC precipitates, leading to performance degradation and strong reflection signals during ultrasonic testing, resulting in defects that cause low product yield and a scrap rate exceeding 15%. To address the shortcomings of existing technologies, this invention provides a method for preparing blade steel and the blade steel itself, aiming to suppress the formation of coarse NbC, eliminate abnormal microstructure, and ultimately improve the mechanical properties, flaw detection yield, and corrosion resistance uniformity of the steel.

[0025] According to one aspect of the present invention, a method for preparing blade steel is provided. For example... Figure 1 As shown, the method mainly includes the following steps: Step S1: The alloy raw materials are melted by vacuum magnetic induction. After the melting is qualified, the molten steel is cast into electrode rods in a vacuum environment. After the electrode rods are formed, ultrasonic flaw detection is performed, and the qualified electrode rods are selected. Step S2: Place the qualified electrode rod as a consumable electrode into the electroslag remelting furnace for electroslag remelting, control the temperature of the molten pool and the remelting speed, and solidify it into a billet in the crystallizer. When the billet temperature drops below the first temperature, remove the billet from the crystallizer and then slowly cool it to room temperature at a rate not exceeding 20℃ / min. Step S3: The billet is heated to a second temperature below the NbC melting temperature using a segmented heating system, and then forged in multiple passes. The deformation amount of the first pass is 15%~20%, the deformation amount of each intermediate pass is 12%~18%, and the deformation amount of the final forging pass is more than 20% and the total forging deformation amount is more than 70%. Step S4: After forging is completed, the forging is cooled in a controlled manner. First, it is cooled at a high cooling rate to 650~700℃, then cooled at a low cooling rate to below 300℃, and then air-cooled to room temperature. The blade steel comprises the following components by mass fraction: C: 0.05%~0.1%; Nb: 0.08%~0.1%; Cr: 13.8%~15.0%; Ni: 5.5%~7.0%; Mo: 1.3%~1.6%; Cu: 1.9%~2.4%; with the balance being Fe and unavoidable impurities.

[0026] By optimizing the composition of the blade steel, especially the content range of C and Nb, the product of the solid solution concentrations of C and Nb in the steel is controlled within a critical range that can form sufficient NbC for strengthening without causing NbC coarsening, thereby suppressing the precipitation of coarse NbC. During vacuum magnetic induction smelting, appropriate smelting temperatures are used in conjunction with electromagnetic stirring and argon bottom blowing to avoid Nb element loss and ensure the uniformity of the steel composition. During electroslag remelting and purification smelting, by controlling the molten pool temperature, remelting speed, slag composition, and cooling method, local segregation of Nb element is suppressed and the internal stress of the billet is reduced, thus improving the quality of the billet. By optimizing the forging process (including heating, deformation, and cooling) and post-forging cooling control, a "multi-pass, small deformation" forging strategy is adopted to break down and refine NbC, while introducing high-density dislocations to inhibit NbC growth. During post-forging cooling, the cooling is controlled to quickly pass through the NbC precipitation sensitive area, reducing the chance of NbC precipitation.

[0027] The blade steel prepared using the method of this invention possesses excellent mechanical and corrosion resistance properties. Its hardness reaches 328-336 HV, tensile strength ≥900 MPa, yield strength ≥730 MPa, and retained austenite volume fraction is less than 1%. In a neutral salt spray test (5% NaCl solution, 35℃), the corrosion rate is ≤0.01 mm / year, meeting the long-term safe service requirements of ultra-supercritical generator blades under high temperature, high pressure, high stress, and corrosive environments. Furthermore, the NbC size in the blade steel does not exceed 4 μm and is diffusely distributed, effectively avoiding problems such as uneven hardness, unqualified flaw detection, and localized corrosion caused by coarse, elongated NbC precipitates.

[0028] The following is a detailed description of each step of the method for preparing blade steel according to the present invention.

[0029] In step S1, the alloy raw materials are melted by vacuum magnetic induction. After the melting is qualified, the molten steel is cast into electrode rods in a vacuum environment. After the electrode rods are formed, ultrasonic flaw detection is performed, and the qualified electrode rods are selected.

[0030] The blade steel is formulated according to the following components by mass fraction: C: 0.05%~0.1%; Nb: 0.08%~0.1%; Cr: 13.8%~15.0%; Ni: 5.5%~7.0%; Mo: 1.3%~1.6%; Cu: 1.9%~2.4%, with the balance being Fe and unavoidable impurities.

[0031] Traditional martensitic blade steel does not coordinate the content of C and Nb, but only controls the content of the control element. This leads to the product of the solid solution concentrations of C and Nb in the steel ([C]×[Nb]) possibly exceeding the critical value. Nb and C preferentially aggregate in a "non-dispersive nucleation" manner, forming long strips of coarse NbC with a size of over 10μm. This invention synergistically controls the content of C and Nb, keeping the C content between 0.05% and 0.1% and the Nb content between 0.08% and 0.1%. Through thermodynamic calculations and experimental verification, the solid solution concentration product of C and Nb ([C]×[Nb]∈0.004%~0.01%) is in a critical range that can form sufficient NbC for strengthening without causing NbC coarsening. Within this range, Nb and C will precipitate NbC in a "dispersed nucleation" manner, which avoids coarsening due to excessively high [C]×[Nb] and prevents insufficient strengthening due to excessively low [C]×[Nb].

[0032] In blade steel, the carbon content is controlled at 0.05%~0.1%. This range provides basic hardness to the martensitic matrix and combines with Nb to form the NbC strengthening phase. If the content is too low, the martensitic hardness will be insufficient; if it is too high, it will easily combine excessively with Nb, leading to NbC coarsening. Therefore, it needs to be controlled in conjunction with Nb.

[0033] The Nb content in the blade steel is controlled at 0.08%~0.1%, which matches the C content to ensure that the product of the solid solution concentrations of the two is within the critical range. Combined with the toughness requirements of the blade steel, its content is finally controlled at 0.08%~0.1%.

[0034] Adding 13.8%~15.0% Cr to blade steel to form a Cr2O3 passivation film on the surface serves as the core guarantee for corrosion resistance, which can resist corrosive media such as oil, gas, and steam, and prevent oxidation or pitting on the blade surface.

[0035] Adding 5.5% to 7.0% Ni to blade steel can improve the toughness of the martensitic matrix and suppress low-temperature brittleness; at the same time, it can stabilize a small amount of residual austenite, balance the hardness and plasticity of the steel, and prevent the blade from breaking under alternating loads.

[0036] Adding 1.3% to 1.6% Mo to blade steel can enhance the stability of the passivation film, especially improving its resistance to Cl. - S - It enhances resistance to corrosive ions, refines grain size, and improves the uniformity of NbC distribution.

[0037] Adding 1.9% to 2.4% Cu to blade steel can form a Cu-rich dispersed precipitate phase, which works synergistically with NbC to achieve dual strengthening, further improving the tensile strength of the steel without sacrificing corrosion resistance.

[0038] The process utilizes high-purity base alloys, including low-carbon steel, Cr raw materials with a purity ≥99.9%, Ni raw materials with a purity ≥99.95%, Mo raw materials with a purity ≥99.9%, Cu raw materials with a purity ≥99.9%, and high-purity FeNb alloy. The FeNb alloy has a purity ≥99.8%, Si ≤0.01%, and Al ≤0.005%. By controlling the purity of the raw materials, the introduction of impurities is reduced, ensuring the accuracy and stability of the chemical composition of the blade steel, laying a solid foundation for subsequent smelting and processing. A fully automated batching system is preferred for proportionally adding FeNb, ensuring that the Nb content in the finished molten steel is precisely controlled between 0.08% and 0.1%; simultaneously, the C content is controlled between 0.05% and 0.1%, reducing the raw material basis for excessive C-Nb combination to form coarse NbC.

[0039] In some embodiments of this invention, the raw materials are added to a vacuum magnetic induction furnace, and a vacuum is drawn after the furnace door is closed to prevent the intrusion of gases such as O and N during the melting process. The presence of N promotes the coarsening of NbC because NbN precipitates more easily in steel than NbC. The NbN particles that precipitate first act as nucleation sites for heterogeneous formation. During subsequent cooling or tempering, NbC precipitates and grows attached to these nuclei, causing what might have been individually small and dispersed NbC to become composite carbonitrides with NbN as the core. These composites are often much larger than pure NbC and have irregular shapes. Therefore, this invention uses a vacuum magnetic induction furnace for preliminary melting and performs a vacuuming operation before melting to strictly control the N in the melting environment. The temperature is raised to 1580~1620℃ during melting. Setting the melting temperature at this temperature, which is 50~80℃ higher than the liquidus line of molten steel and lower than the Nb volatilization temperature threshold, ensures complete dissolution of the alloy while preventing the loss of N.

[0040] In some embodiments of the present invention, during vacuum magnetic induction melting, the electromagnetic stirring device is activated at a stirring frequency of 3-5 Hz, and argon gas with a purity ≥99.999% is introduced for bottom blowing at a flow rate of 0.8-1.2 L / min, with continuous stirring for 40-60 min. Electromagnetic stirring forces the melt to flow, breaks up dendrites, significantly reduces the microscopic segregation of Nb, and ensures a more uniform distribution of Nb, C, and other elements at a more microscopic scale. The bottom-blowing argon gas allows the argon bubbles to not only carry away harmful gases dissolved in the molten steel (such as nitrogen and hydrogen) during their ascent, but also adsorb and float to remove tiny inclusions in the molten steel, further enhancing the fluidity of the melt. Especially in dead zones or deep areas of the melt that may exist with electromagnetic stirring, argon stirring effectively promotes melt circulation, ensuring a highly uniform composition and temperature throughout the molten pool, completely eliminating localized Nb enrichment zones, and preventing subsequent concentrated precipitation of NbC.

[0041] In some embodiments of the present invention, the casting of the electrode rod is controlled as follows: after the smelting is qualified, the molten steel is cast into an electrode rod in a vacuum environment (≤10Pa). The casting temperature is controlled at 1530~1550℃ and the cooling rate is ≤15℃ / min to avoid shrinkage cavities and cracks inside the electrode rod due to excessive cooling, or premature precipitation of NbC induced by local overcooling. After the electrode rod is formed, it needs to be ultrasonically tested (pass rate ≥99%) to remove billets with loose or mixed contents and select qualified electrode rods.

[0042] Since the subsequent electroslag remelting operation is essentially a micro-region sequential solidification process with a slow melting rate and limited liquid phase mass transfer, the compositional uniformity of the electrode rod will affect the quality of the electroslag remelted steel billet. Electrode rods with uniform composition and few defects are a prerequisite for obtaining high-quality electroslag remelted steel billets.

[0043] In step S2, a qualified electrode rod is placed in an electroslag remelting furnace as a consumable electrode for electroslag remelting. The temperature of the molten pool and the remelting speed are controlled. The molten steel is directionally solidified into a billet in a crystallizer. When the billet temperature drops below the first temperature, the billet is removed from the crystallizer and then slowly cooled to room temperature at a rate not exceeding 20°C / min. Electroslag remelting can achieve purification, densification of the billet, and reduction of Nb segregation.

[0044] In some embodiments of the present invention, the slag system used for electroslag remelting is a ternary slag system of CaF2-CaO-Al2O3, wherein the mass fraction of Al2O3 is 5%~10%, the mass fraction of CaO is 15%~20%, and the balance is CaF2. The slag material is pretreated as follows: dried at 1200℃ for more than 4 hours to completely remove moisture, prevent the generation of H2 gas during remelting, and avoid the formation of bubble defects; the slag material is crushed to a particle size of 5~15mm to ensure uniform melting of the slag system.

[0045] In some embodiments of the present invention, a qualified electrode rod prepared by vacuum magnetic induction melting is used as a consumable electrode and vertically placed in the water-cooled copper crystallizer of an electroslag remelting furnace. After arc ignition, the open-circuit voltage is adjusted to 65-75V and the working current to 3500-4500A to maintain the molten pool temperature stable at 1600-1650℃, avoiding local overheating that could cause Nb element segregation along grain boundaries, while ensuring that the slag system is fully melted, and removing non-metallic inclusions in the steel through "slag washing". During the remelting process, a vacuum is continuously pumped to maintain the vacuum degree in the furnace at ≤20Pa to prevent air from entering and causing secondary oxidation and the formation of NbN and its complexes.

[0046] The electroslag remelting pool temperature of this invention is controlled between 1600 and 1650°C. This temperature window is a range obtained through precise calculation and optimization based on the specific alloy composition system of this invention, and corresponds to the alloy composition and the content of each element. Different alloy element contents will significantly change the liquidus temperature of the steel. The pool temperature must ensure that the slag system can be fully melted and has a suitable viscosity. Controlling the pool temperature between 1600 and 1650°C ensures good slag fluidity, which can fully encapsulate non-metallic inclusions in the molten steel and achieve efficient removal; it also controls the thermal gradient at the solidification front, preventing the formation of coarse primary NbC due to local overheating and Nb enrichment between dendrites.

[0047] During the billet forming and slow cooling process, the remelting speed is controlled at 0.4~0.8 m / min. Directional solidification is achieved through a water-cooled copper crystallizer, ensuring uniform axial crystallization of the billet and reducing Nb dendrite segregation. In some embodiments of this invention, the inlet water temperature of the water-cooled copper crystallizer is controlled at 25~30℃, and the outlet water temperature is ≤45℃.

[0048] In some embodiments of the present invention, after remelting, when the temperature of the billet drops to below a preset temperature selected from 550~600°C, it is moved into a slow cooling pit and slowly cooled to room temperature at a rate of ≤20°C / min to avoid internal stress caused by excessive temperature difference between the inside and outside of the billet, and to prevent NbC precipitation caused by rapid cooling.

[0049] In step S3, a segmented heating system is used to heat the billet to a second temperature below the NbC melting temperature for multi-pass forging. The deformation amount of the first pass is 15% to 20%, the deformation amount of each intermediate pass is 12% to 18%, the deformation amount of the final forging pass is more than 20%, and the total forging deformation amount is more than 70%.

[0050] In some embodiments of the present invention, the segmented heating regime includes: heating from room temperature to 600-700°C at a heating rate of 80-100°C / h, holding at that temperature for 1-1.5 hours; then heating to 1150-1200°C at a heating rate of 50-70°C / h, holding at that temperature for 2.5-3.5 hours. The initial heating rate of 80-100°C / h from room temperature to 600-700°C rapidly crosses the low-temperature brittleness range, reducing the risk of cracking and preparing for subsequent phase transformation and element diffusion. Holding at 600-700°C for 1-1.5 hours allows the surface temperature of the cast billet to pause its rise, allowing the core temperature to gradually increase, thereby reducing the cross-sectional temperature difference and thermal stress caused by rapid heating. The subsequent heating rate of 50-70°C / h to 1150-1200°C ensures uniform cross-sectional temperature. The billet is held at 1150~1200℃ for 2.5~3.5 hours to ensure complete austenitization of the microstructure. This temperature is also below the NbC melting temperature of 1220~1250℃ to prevent excessive dissolution and re-coarsening of fine NbC. The holding time is adjusted according to the billet thickness to ensure uniform temperature throughout the billet and that the temperature remains below the NbC melting temperature.

[0051] Multi-pass forging aims to break up potential NbC, introduce dislocations to inhibit growth, and adopt a forging strategy of "multi-pass, small deformation".

[0052] In some embodiments of the present invention, the deformation amount of the first forging pass is 15% to 20% to utilize sufficient deformation force to break up the residual fine NbC particles in the billet, while simultaneously breaking the columnar crystal structure in the as-cast state. The intermediate forging passes may include 2 to 4 passes, with a deformation amount of 12% to 18% per pass, and the forging temperature is controlled at 980 to 1080°C. Through continuous deformation, a high density of dislocations is introduced into the austenite matrix. These dislocations can act as "dispersed nucleation sites" for NbC precipitation, preventing the aggregation of Nb atoms to form coarse NbC. The deformation amount per single pass should not be too large, as excessive deformation will lead to a drastic temperature rise, which will instead promote carbide coarsening; too small a deformation will fail to effectively break up NbC. The final forging pass controls the final forging temperature at 890 to 960°C, with a final forging deformation amount ≥20%. The total forging deformation amount is ≥70%, ensuring thorough microstructure refinement, eliminating internal porosity in the billet, and simultaneously inhibiting NbC growth.

[0053] In step S4, after forging is completed, the forging is cooled in a controlled manner. First, it is cooled at a high cooling rate to 650~700℃, then at a lower cooling rate to 250~300℃, and then air-cooled to room temperature.

[0054] In some embodiments of the present invention, controlled cooling includes: after forging, immediately transferring the forging into a controlled cooling device, first cooling it to 650-700°C at a cooling rate of 30-50°C / min, then cooling it to 250-300°C at a cooling rate of 10-20°C / min, and finally air cooling to room temperature. Cooling to 650-700°C at a rate of 30-50°C / min quickly passes through the NbC precipitation-sensitive zone, reducing the chance of NbC precipitation. Cooling to 250-300°C at a rate of 10-20°C / min prevents the forging from cracking due to excessively rapid cooling, while also preventing NbC coarsening caused by slow cooling.

[0055] In some embodiments of the present invention, if the forging requires subsequent machining or further heat treatment, stress-relief annealing can be performed after air cooling to room temperature following forging. The annealing process includes heating the forging to 550-600°C, holding it at that temperature for 1.5-2.5 hours, furnace cooling to 300°C, and then air cooling to room temperature. This process eliminates forging internal stress, and since the temperature is below the NbC precipitation temperature, it does not induce the formation of coarse NbC.

[0056] The method of this invention completely eliminates the abnormal structure of coarse NbC. Through the coordinated control of the entire process of "smelting (combination of vacuum magnetic induction and electroslag remelting) - forging", the microstructure of the finished steel is lath martensite + fine dispersed NbC (size ≤4μm), without coarse long strip NbC, and the volume fraction of austenite is ≤1%, completely eliminating the abnormal structure of "coarse NbC + 92% martensite + 8% austenite". The ultrasonic flaw detection pass rate is increased to over 99.5% (far higher than the ≤85% of the existing process). The composition and mechanical properties meet the standards. The Nb content of the finished steel is stable at 0.03%~0.05%, the hardness reaches 328~336HV, the tensile strength is ≥900MPa, and the yield strength is ≥730MPa. The mechanical properties are improved by 18%~20% compared with the products of the existing process. In the neutral salt spray test (5% NaCl solution, 35℃), the corrosion rate is ≤0.01mm / year.

[0057] According to another aspect of the present invention, a blade steel is also provided, which is prepared by means of the method described in any of the above embodiments.

[0058] Furthermore, the blade steel contains NbC with a size not exceeding 4μm and a diffuse distribution, has a hardness of 328~336HV, a tensile strength ≥900MPa, a yield strength ≥730MPa, and a retained austenite volume fraction of less than 1%.

[0059] The method of the present invention will be further described and illustrated below with reference to embodiments.

[0060] Example 1 The blade steel to be prepared in this embodiment has the following composition: C: 0.08%; Nb: 0.09%; Cr: 14.4%; Ni: 6.5%; Mo: 1.45%; Cu: 2.2%, with the balance being Fe and unavoidable impurities.

[0061] Vacuum magnetic induction melting: Add the raw materials to the vacuum magnetic induction furnace, close the furnace door and evacuate to 8Pa to prevent the intrusion of gases such as O and N during the melting process; heat to 1600℃, turn on the electromagnetic stirring device, the stirring frequency is 4Hz, and argon gas with a purity ≥99.999% is introduced for bottom blowing, the argon gas flow rate is 1.0L / min, and stirring is continued for 50min; Electrode rod casting control: After the melting is qualified, the molten steel is cast into forging electrode rods in a vacuum environment (≤10Pa), the casting temperature is controlled at 1540℃, vacuum casting is Φ320mm billet, the cooling rate is 10℃ / min, and the electrode rods need to be ultrasonically inspected after forming to remove billets with loose or inclusion-containing contents.

[0062] Electroslag remelting purification: The slag system used in electroslag remelting is a ternary slag system of CaF2-CaO-Al2O3, in which the mass fraction of Al2O3 is 8%, the mass fraction of CaO is 17%, and the balance is CaF2. The pretreatment of the slag is as follows: drying at 1200℃ for more than 4 hours to completely remove moisture, and crushing the slag to a particle size of 10mm; using qualified electrode rods prepared by vacuum magnetic induction melting as consumable electrodes, vertically placing them into the water-cooled copper crystallizer of the electroslag remelting furnace, and adjusting the open-circuit voltage to 70V and the working current to 4000A after arc ignition. To maintain a stable molten pool temperature of 1630℃, a vacuum was continuously applied during remelting to maintain a vacuum level of 15Pa inside the furnace, preventing air from entering and causing secondary oxidation. For billet forming and slow cooling: the remelting pulling speed was controlled at 0.6m / min, and the inlet water temperature of the water-cooled copper crystallizer was controlled at 28℃ and the outlet water temperature at 35℃ to achieve directional solidification, so that the billet crystallizes uniformly along the axial direction and reduces dendritic segregation of Nb element. After remelting, when the billet temperature drops to 580℃, it is moved into the slow cooling pit and slowly cooled to room temperature at a rate of 15℃ / min.

[0063] Pre-forging heating: The electroslag remelted billet is fed into a walking beam furnace and a segmented heating regime is adopted: the temperature is raised from room temperature to 650℃ at a heating rate of 90℃ / h and held for 1.2h; then it is raised to 1180℃ at a heating rate of 60℃ / h and held for 3h.

[0064] Multi-pass forging: The first pass has a deformation of 17%; the intermediate three passes have deformations of 16%, 15%, and 14% respectively, with the forging temperature controlled at 1000℃. Through continuous deformation, high-density dislocations are introduced into the austenite matrix. These dislocations can serve as "dispersed nucleation sites" for NbC precipitation, preventing Nb atoms from agglomerating to form coarse NbC. The final forging pass controls the final forging temperature at 940℃, with a final forging deformation of 21%.

[0065] Post-forging cooling: The forging is immediately transferred to a controlled cooling device and cooled to 680°C at a rate of 40°C / min, then to 290°C at a rate of 15°C / min, and finally air-cooled to room temperature.

[0066] Figure 2 The image shows an SEM image of the blade steel prepared according to Example 1 of the present invention. Its matrix is ​​lath martensite with fine and dispersed NbC strengthening phases, without elongated or blocky coarsening morphology, and without obvious microstructure segregation.

[0067] Performance indicators: hardness 330HV, tensile strength 908MPa, yield strength 735MPa; withstands continuous operation in a 450℃ steam environment for more than 100 hours; ultrasonic flaw detection pass rate 99.6%.

[0068] Application results: After 150 hours of service at 450℃ under steam medium conditions, the blade sealing surface showed no excessive wear and the metallographic structure showed no obvious aging, meeting the dual requirements of corrosion resistance and strength for high-pressure turbine blades.

[0069] Example 2 The blade steel to be prepared in this embodiment has the following composition: C: 0.1%; Nb: 0.08%; Cr: 15.0%; Ni: 7.0%; Mo: 1.6%; Cu: 2.4%, with the balance being Fe and unavoidable impurities.

[0070] Vacuum magnetic induction melting: Add the raw materials to the vacuum magnetic induction furnace, close the furnace door and evacuate to prevent O, N and other gases from entering during the melting process; heat to 1620℃, turn on the electromagnetic stirring device, the stirring frequency is 5Hz, and argon gas with a purity ≥99.999% is introduced for bottom blowing, the argon gas flow rate is 1.2L / min, and stirring is continued for 60min; Electrode rod casting control: After the melting is qualified, the molten steel is cast into forging electrode rods in a vacuum environment (≤10Pa), the casting temperature is controlled at 1550℃, the cooling rate is 15℃ / min, and the electrode rods need to be ultrasonically inspected after forming to remove billets with loose or inclusion-containing materials.

[0071] Electroslag remelting purification: The slag system used in electroslag remelting is a ternary slag system of CaF2-CaO-Al2O3, in which the mass fraction of Al2O3 is 10%, the mass fraction of CaO is 20%, and the balance is CaF2. The pretreatment method of the slag material is as follows: drying at 1200℃ for more than 4 hours to completely remove moisture, and crushing the slag material to a particle size of 15mm; using qualified electrode rods prepared by vacuum magnetic induction melting as consumable electrodes, vertically placing them into the water-cooled copper crystallizer of the electroslag remelting furnace, and adjusting the open-circuit voltage to 75V and the working current to 4500 after arc ignition. A. Maintain a stable molten pool temperature of 1650℃. During the remelting process, continuously apply vacuum to maintain a vacuum level of 20Pa inside the furnace to prevent air from entering and causing secondary oxidation. For billet forming and slow cooling: control the remelting pulling speed at 0.8m / min, and control the inlet water temperature of the water-cooled copper crystallizer at 30℃ and the outlet water temperature at 45℃ to achieve directional solidification, so that the billet crystallizes uniformly along the axial direction and reduces dendritic segregation of Nb element. After remelting, when the billet temperature drops to 600℃, move it into the slow cooling pit and slow cool it to room temperature at a rate of 20℃ / min.

[0072] Pre-forging heating: The electroslag remelted billet is fed into a walking beam furnace and a segmented heating regime is adopted: the temperature is raised from room temperature to 700℃ at a heating rate of 100℃ / h and held for 1 hour; then it is raised to 1200℃ at a heating rate of 70℃ / h and held for 2.5 hours.

[0073] Multi-pass forging: The first pass has a deformation of 20%; four intermediate forging passes are performed, with deformation amounts of 18%, 15%, 14%, and 13% respectively, and the forging temperature is controlled at 1080℃. Through continuous deformation, high-density dislocations are introduced into the austenite matrix. Dislocations can serve as "dispersed nucleation sites" for NbC precipitation, preventing Nb atoms from agglomerating to form coarse NbC; the final forging pass controls the final forging temperature at 960℃, with a final forging deformation of 23%.

[0074] Post-forging cooling: The forging is immediately moved into a controlled cooling device and cooled to 700°C at a rate of 50°C / min, then to 300°C at a rate of 20°C / min, and finally air-cooled to room temperature.

[0075] Microstructure: The steel matrix of this blade is lath martensite with fine, dispersed NbC strengthening phases, without elongated or blocky coarsening morphology, and without obvious microstructure segregation. Performance indicators: Hardness 329 HV, tensile strength 905 MPa, yield strength 733 MPa; withstands over 100 hours in a 450℃ steam environment; ultrasonic flaw detection pass rate 99.5%. Application results: After 150 hours of service in a 450℃ steam medium environment, the blade sealing surface showed no excessive wear, and the metallographic structure showed no obvious aging, meeting the dual requirements of corrosion resistance and strength for high-pressure turbine blades.

[0076] Example 3 The blade steel to be prepared in this embodiment has the following composition: C: 0.05%; Nb: 0.1%; Cr: 13.8%; Ni: 5.5%; Mo: 1.3%; Cu: 1.9%, with the balance being Fe and unavoidable impurities.

[0077] Vacuum magnetic induction melting: Add the raw materials to the vacuum magnetic induction furnace, close the furnace door and evacuate to prevent O, N and other gases from entering during the melting process; heat to 1580℃, turn on the electromagnetic stirring device, the stirring frequency is 3Hz, and argon gas with a purity ≥99.999% is introduced for bottom blowing, the argon gas flow rate is 0.8L / min, and stirring is continued for 40min; Electrode rod casting control: After the melting is qualified, the molten steel is cast into forging electrode rods in a vacuum environment (≤10Pa), the casting temperature is controlled at 1530℃, the cooling rate is 12℃ / min, and the electrode rods need to be ultrasonically inspected after forming to remove billets with loose or inclusion-containing materials.

[0078] Electroslag remelting purification: The slag system used in electroslag remelting is a ternary slag system of CaF2-CaO-Al2O3, in which the mass fraction of Al2O3 is 5%, the mass fraction of CaO is 15%, and the balance is CaF2. The pretreatment of the slag is as follows: drying at 1200℃ for more than 4 hours to completely remove moisture, and crushing the slag to a particle size of 5mm; using qualified electrode rods prepared by vacuum magnetic induction melting as consumable electrodes, vertically placing them into the water-cooled copper crystallizer of the electroslag remelting furnace, and adjusting the open-circuit voltage to 65V and the working current to 3500A after arc ignition. To maintain a stable molten pool temperature of 1600℃, a vacuum was continuously applied during the remelting process to maintain a vacuum level of 18Pa inside the furnace, preventing air from entering and causing secondary oxidation. For billet forming and slow cooling: the remelting pulling speed was controlled at 0.4m / min, and the inlet water temperature of the water-cooled copper crystallizer was controlled at 25℃ and the outlet water temperature at 35℃ to achieve directional solidification, so that the billet crystallizes uniformly along the axial direction and reduces dendritic segregation of Nb element. After remelting, when the billet temperature drops to 550℃, it is moved into the slow cooling pit and slowly cooled to room temperature at a rate of 18℃ / min.

[0079] Pre-forging heating: The electroslag remelted billet is fed into a walking beam furnace and a segmented heating regime is adopted: the temperature is raised from room temperature to 600℃ at a heating rate of 80℃ / h and held for 1.5h; then it is raised to 1150℃ at a heating rate of 50℃ / h and held for 3.5h.

[0080] Multi-pass forging: The first pass has a deformation of 15%; the intermediate two passes have deformations of 18% and 17% respectively, with the forging temperature controlled at 980℃. Through continuous deformation, high-density dislocations are introduced into the austenite matrix. These dislocations can serve as "dispersed nucleation sites" for NbC precipitation, preventing Nb atoms from agglomerating to form coarse NbC. The final forging pass controls the final forging temperature at 890℃ and the final forging deformation at 25%.

[0081] Post-forging cooling: The forging is immediately transferred to a controlled cooling device and cooled to 650°C at a rate of 30°C / min, then to 250°C at a rate of 10°C / min, and finally air-cooled to room temperature.

[0082] Comparative Example 1 The blade steel to be prepared in this comparative example has the following composition: C: 0.23%; Nb: 0.20%; Cr: 14%; Ni: 5.5%; Mo: 1.5%; Cu: 1.8%, with the balance being Fe and unavoidable impurities.

[0083] Vacuum magnetic induction melting: Add the raw materials to the vacuum magnetic induction furnace, close the furnace door and evacuate to prevent O, N and other gases from entering during the melting process; heat to 1650℃, introduce argon gas with a purity ≥99.999% for bottom blowing, the argon gas flow rate is 1.0L / min, and stir continuously for 50min; Electrode rod casting control: After the melting is qualified, cast the molten steel into forging electrode rods in a vacuum environment (≤10Pa), the casting temperature is controlled at 1540℃, vacuum casting is used to form Φ320mm billets, and the cooling rate is 20℃ / min.

[0084] Electroslag remelting purification: The slag system used for electroslag remelting is a ternary slag system of CaF2-CaO-Al2O3, in which the mass fraction of Al2O3 is 8%, the mass fraction of CaO is 17%, and the balance is CaF2. The slag material is pretreated and crushed to a particle size of 10mm. Qualified electrode rods prepared by vacuum magnetic induction melting are used as consumable electrodes and vertically placed into the water-cooled copper crystallizer of the electroslag remelting furnace to maintain the molten pool temperature at a stable 1680℃. Casting and cooling: The remelting pulling speed is controlled at 1m / min, and the inlet water temperature of the water-cooled copper crystallizer is controlled at 20℃ and the outlet water temperature is controlled at 35℃. After remelting, when the temperature of the cast billet drops to 580℃, it is cooled to room temperature at a rate of 25℃ / min.

[0085] Pre-forging heating: The electroslag remelted billet is heated to 1250℃ at a heating rate of 60℃ / h and held for 5h.

[0086] Forging: The first forging pass has a deformation amount of 12%; the intermediate forging passes have two forging passes with deformation amounts of 14% and 13% respectively; the final forging pass has a final forging temperature of 940℃ and a final forging deformation amount of 15%.

[0087] Post-forging cooling: The forging is immediately moved into a controlled cooling device and cooled to 290°C at a rate of 15°C / min, and then air-cooled to room temperature.

[0088] The microstructure of the blade steel prepared in this comparative example is as follows: Figure 3As shown in the figure, Figure (a) is a SEM image magnified 100 times. It can be seen that coarse, elongated NbC precipitates (indicated by arrows) and abnormal matrix structures appear in the matrix. The abnormal matrix structure surrounding the coarse, elongated NbC precipitates is brighter than the normal matrix. This is because the formation of the coarse, elongated NbC precipitates reduces the C content of the surrounding matrix. A decrease in matrix C content leads to a decrease in matrix hardness, which is approximately 195 HV. This results in uneven corrosion resistance and failure to pass flaw detection. Figure (b) is a SEM image magnified 5000 times, showing a magnified view of the coarse, elongated NbC precipitates and the surrounding abnormal matrix. Figure (c) is a Nb elemental surface scan, confirming that the coarse, elongated precipitates are NbC. Figure (d) shows the EBSD test results. The red part in the figure is martensite, and the green part is retained austenite. The analysis shows that the matrix is ​​about 92% martensite and 8% retained austenite. The excessive content of retained austenite will directly reduce the overall surface hardness of the blade steel, reduce its wear resistance and erosion resistance. Moreover, retained austenite is a metastable phase, which can easily lead to unstable mechanical properties of blade steel at high temperatures, resulting in problems such as deformation.

[0089] 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 preparing blade steel, characterized in that, Includes the following steps: The alloy raw materials are melted by vacuum magnetic induction. After the melting is qualified, the molten steel is cast into electrode rods in a vacuum environment. After the electrode rods are formed, ultrasonic flaw detection is performed, and the qualified electrode rods are selected. Qualified electrode rods are placed in an electroslag remelting furnace as consumable electrodes for electroslag remelting. The temperature of the molten pool and the remelting speed are controlled. The molten pool is directionally solidified into a billet in the crystallizer. When the billet temperature drops below the first temperature, the billet is removed from the crystallizer and then slowly cooled to room temperature at a rate not exceeding 20°C / min. The billet is heated to a second temperature below the NbC melting temperature using a segmented heating system, and then forged in multiple passes. The deformation amount in the first pass is 15% to 20%, the deformation amount in each of the intermediate passes is 12% to 18%, and the deformation amount in the final forging pass is more than 20% and the total forging deformation amount is more than 70%. After forging, the forging is cooled in a controlled manner. First, it is cooled at a high cooling rate to 650~700℃, then at a lower cooling rate to 250~300℃, and then air-cooled to room temperature. The blade steel comprises the following components by mass fraction: C: 0.05%~0.1%; Nb: 0.08%~0.1%; Cr: 13.8%~15.0%; Ni: 5.5%~7.0%; Mo: 1.3%~1.6%; Cu: 1.9%~2.4%; balance is Fe and unavoidable impurities.

2. The method for preparing blade steel according to claim 1, characterized in that, The alloy raw material includes FeNb alloy, wherein the FeNb alloy has a purity ≥99.8%, Si ≤0.01%, and Al ≤0.005%.

3. The method for preparing blade steel according to claim 1, characterized in that, The temperature of the vacuum magnetic induction melting is 1580~1620℃. During the melting process, the electromagnetic stirring device is turned on, the stirring frequency is 3~5Hz, and argon gas with a purity ≥99.999% is introduced for bottom blowing, the argon gas flow rate is 0.8~1.2L / min, and stirring is continued for 40~60min.

4. The method for preparing blade steel according to claim 1, characterized in that, The casting temperature is 1530~1550℃, and the casting process is cooled at a rate of less than 15℃ / min.

5. The method for preparing blade steel according to claim 1, characterized in that, The slag system used in the electroslag remelting is a ternary slag system of CaF2-CaO-Al2O3, wherein the mass fraction of Al2O3 is 5%~10%, the mass fraction of CaO is 15%~20%, and the balance is CaF2.

6. The method for preparing blade steel according to claim 1, characterized in that, The parameters for the electroslag remelting are set as follows: After arc ignition, adjust the open-circuit voltage to 65~75V, the working current to 3500~4500A, and maintain the molten pool temperature at 1600~1650℃. Maintain a vacuum level of ≤20Pa inside the furnace during the remelting process.

7. The method for preparing blade steel according to claim 1, characterized in that, The remelting speed is controlled at 0.4~0.8 m / min, the inlet water temperature of the crystallizer is 25~30℃, the outlet water temperature is ≤45℃, and the first temperature is 550~600℃.

8. The method for preparing blade steel according to claim 1, characterized in that, The segmented heating regime includes: heating from room temperature to 600-700℃ at a heating rate of 80-100℃ / h, holding at that temperature for 1-1.5h; then heating to 1150-1200℃ at a heating rate of 50-70℃ / h, holding at that temperature for 2.5-3.5h. The intermediate passes are forged in 2 to 4 passes at a forging temperature of 980 to 1080°C. The forging temperature of the final forging pass is controlled at 890~960℃.

9. The method for preparing blade steel according to claim 1, characterized in that, The controlled cooling includes: after forging, the forging is immediately moved into a controlled cooling device, first cooled to 650-700°C at a cooling rate of 30-50°C / min, then cooled to 250-300°C at a cooling rate of 10-20°C / min, and finally air-cooled to room temperature; or The method further includes annealing the forging after controlled cooling. The annealing process includes heating the forging to 550~600℃, holding it at that temperature for 1.5~2.5h, cooling it in the furnace to 300℃, and then air cooling it to room temperature.

10. A blade steel, characterized in that, The blade steel is prepared by any one of claims 1-9, wherein the blade steel contains NbC with a size not exceeding 4 μm and in a diffuse distribution, has a hardness of 328~336HV, a tensile strength ≥900MPa, a yield strength ≥730MPa, and a retained austenite volume fraction of less than 1%.

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