Preparation method of ultrahigh-strength steel member with fatigue impact resistance

Abstract

The invention discloses a preparation method of an ultrahigh-strength steel member with fatigue impact resistance, which successfully realizes organic unification of ultralow roughness, high steady-state gradient compressive stress and high uniform surface energy on the surface of ultrahigh-strength steel through full-chain coordinated regulation and control. The constructed residual stress field is generated in situ by a self-generated heat-force coupling process induced by laser irradiation no longer depending on external plastic deformation, and is cured into a thermodynamic metastable state through a subsequent multi-field synergistic effect, so that the retention capability of the residual stress field in a service temperature change environment is remarkably improved; meanwhile, by means of oriented reconstruction of a molecular-scale surface energy field, anisotropy of surface free energy is eliminated, the preferential adsorption and electrochemical activation tendency of an environmental medium in a stress concentration area is restrained, and the component can present consistent and predictable energy dissipation behaviors when bearing impact loads for the first time; and the fatigue performance stability and the long-term service reliability are fundamentally improved.

Description

Technical Field

[0001] This invention relates to the field of steel component technology, specifically to a method for preparing ultra-high strength steel components with fatigue impact resistance. Background Technology

[0002] In high-end equipment fields such as aerospace, deep-sea engineering, and high-speed rail transportation, ultra-high-strength steel components are subjected to harsh environments with coupled high stress amplitudes and sudden impact loads for extended periods. Current mainstream manufacturing processes generally employ a technical route of conventional smelting, ingot casting, multi-stage forging, tempering heat treatment, precision machining, and surface shot peening. While this route can improve surface compressive stress levels to some extent, it has inherent limitations: the residual compressive stress introduced by shot peening depends on surface plastic deformation, resulting in uneven distribution, steep gradients, and a tendency to be accompanied by increased surface roughness, microcrack initiation, and derivative damage such as amorphous white layers; more importantly, this type of mechanically induced stress has poor thermal stability, exhibiting significant relaxation under slight temperature fluctuations during service, making it difficult to maintain effective suppression of fatigue crack initiation.

[0003] Therefore, the key technical problems faced by existing technologies include: the inability to construct a composite surface state with high amplitude, gentle gradient, thermodynamic stability and uniform surface energy while ensuring ultra-low surface roughness and non-destructive surface structure, resulting in discrete response of components under the first cycle impact load, unpredictable fatigue performance and insufficient service reliability. Summary of the Invention

[0004] This invention aims to provide a method for preparing ultra-high strength steel components with fatigue impact resistance. By reconstructing the surface energy field at the molecular scale, the anisotropy of surface free energy is eliminated, thereby suppressing the preferential adsorption and electrochemical activation tendency of the environmental medium in the stress concentration region.

[0005] To achieve the above objectives, the technical solution adopted by this invention is: a method for preparing ultra-high strength steel components with fatigue impact resistance, comprising: Steel molten material is obtained by vacuum induction melting and dynamic synergistic control of composition. The steel molten material is then subjected to controlled atmosphere electroslag remelting and solidification microstructure gradient refinement to prepare remelted ingots. The remelted ingot is used as a billet to carry out thermo-coupled net near-forming and three-dimensional rheological homogenization to obtain a forging. The forging is then subjected to surface light-quantitative cutting-polishing co-processing and quantitative peeling of the subsurface damage layer to obtain a light-quantitative surface with a surface roughness Ra≤0.08μm. Local laser surface treatment and in-situ construction of gradient compressive stress field are carried out on the optically quantized surface to form a surface layer with residual compressive stress peak value ≥ -860MPa; The surface stress state is solidified in situ and the thermo-mechanical history memory is eliminated to establish a thermodynamic metastable stress field. On the thermodynamic metastable stress field, the surface energy field is reconstructed before service and the adsorption layer is oriented to generate a hydrophobic surface with surface energy isotropy ≥ 0.97. Based on the hydrophobic surface, the full-cycle service status mapping and fatigue impact response pre-calibration are completed, and the output is an ultra-high strength steel component with traceable service status.

[0006] Preferably, the vacuum induction melting and dynamic composition synergistic control include: A low-oxygen melt is obtained by implementing multi-stage pressure gradient degassing and dynamic oxygen potential control in a vacuum induction furnace. The low-oxygen melt was subjected to directional migration and enrichment of sulfur and phosphorus elements and regulation of slag-metal phase distribution. Adding microalloying elements to the melt after sulfur and phosphorus content reduction enables time-sequential addition and solute redistribution equalization based on solidification path constraints. The melt with added microalloying elements was subjected to directional solidification and central densification under multi-scale temperature field synergy to obtain a billet with a central density ≥99.86%.

[0007] Preferably, the controlled atmosphere electroslag remelting and solidification microstructure refinement includes: The initial billet is used as a consumable electrode to implement low-fluorine slag system matching and dynamic compensation control of molten slag resistivity. Under stable slag pool conditions, the molten pool morphology is controlled and the interface stability is enhanced by dual-frequency electromagnetic field coupling. Based on the stable molten pool morphology, solidification structure zoning control is implemented under a radial gradient cooling regime; High-temperature homogenization and grain boundary phase dissolution kinetics control were applied to the remelted ingot with solidification structure control to obtain a homogenized remelted ingot with a grain boundary phase area fraction ≤0.023%.

[0008] Preferably, the thermo-coupled net-close forming and three-dimensional rheological homogenization includes: The homogenized remelted ingot is subjected to multi-stage gradient heating, while the dynamic width of the austenite phase region is identified; when the heated billet is in the single-phase austenite region, closed-loop die forging path planning and metal flow line topology reconstruction are implemented. For forgings after metal flowline reconstruction, triaxial strain tensor equilibration and dynamic recrystallization volume fraction control are performed. A controlled cooling path is implemented after forging for forgings that have undergone dynamic recrystallization, while suppressing the precipitation of proeutectoid phases, ultimately resulting in a net near-formed forging with a grain size of 18.3 μm and no proeutectoid phases.

[0009] Preferably, the surface light-quantitative cutting-polishing co-processing and quantitative removal of the subsurface damage layer include: A stiffness-matching high-speed cutting parameter domain is constructed for the near-net-shape forging, while cutting force closed-loop suppression is implemented. On the surface obtained by cutting, a diamond micro-powder particle size gradient distribution is configured, and an elastic polishing pressure field is designed. Enhance the electrochemical dissolution selectivity in electrolytic-mechanical composite polishing for the polished surface; The electrolytically treated surface is then subjected to ultrasonic-assisted nanofluid final polishing, which promotes the homogenization of surface energy states, ultimately resulting in a light-quantized surface with a subsurface plastic deformation layer depth ≤1.2μm and a surface energy ≤28.1mN / m.

[0010] Preferably, the local laser surface treatment and in-situ construction of the gradient compressive stress field include: The absorption characteristics of the light-quantized surface are analyzed, the spatiotemporal distribution of heat flux density is calculated, and the multi-spot temporal overlapping scanning path is determined. Based on the temperature field after thermal accumulation, the martensite nucleation density is calculated to determine the austenitization-self-tempering two-stage phase transformation path; Based on the microstructure characteristics after phase transformation, the axial residual stress distribution is calculated, and the laser-induced plastic compression parameters are determined. By combining the stress distribution gradient, the stress relaxation rate is calculated, and the stress gradient cutoff and interface stress relaxation suppression process are determined, ultimately obtaining a compressive stress field without stress reversal.

[0011] Preferably, the in-situ curing of surface stress state and elimination of thermo-mechanical history memory include: A dual-temperature zone stepped thermal cycling regime is implemented on the compressive stress field, and a lattice distortion relaxation path is designed. During the thermal cycling heat preservation stage, a micro compressive stress field is synchronously applied to the surface after lattice distortion relaxation to achieve interface stress redistribution. For forgings with completed stress redistribution, martensitic carbon partition stabilization under cooling path constraints is implemented. For forgings with completed carbon partitioning, the thermodynamic metastable state of the stress field is verified, the historical memory effect is eliminated, and a thermodynamic metastable stress field with stress fluctuation amplitude ≤0.12% is obtained.

[0012] Preferably, the pre-service surface energy field reconstruction and adsorption layer orientation arrangement includes: The thermodynamic metastable stress field surface was pretreated with hydroxylation to determine the density of active sites; on the hydroxylated surface, bifunctional silane directional grafting was performed to strengthen Si-O-Fe bonding. Silane-grafted surfaces are subjected to epoxy end-group closed-ring crosslinking to achieve isotropic surface energy. By constructing nanoscale hydrophobic channels on a component with uniform surface energy, a capillary blocking effect is formed, resulting in a hydrophobic surface with an average pore size of 24 nm and no capillary condensation at a relative humidity of 60%.

[0013] Preferably, the full-cycle service state mapping and fatigue shock response pre-calibration includes: Analyze the state parameters of the hydrophobic surface, calculate the mapping relationship between the surface state parameters and fatigue life, calculate the plastic dissipation work in the first cycle based on the fatigue life mapping relationship, and determine the surface state fine-tuning parameters. Based on the surface condition after fine-tuning, the sensitivity of parameter drift to fatigue life is calculated, an early warning threshold is set, and for components with set early warning thresholds, the coefficient of variation of equivalent impact response is calculated multiple times to verify the consistency of response throughout the entire cycle.

[0014] Preferably, the calculation of the surface condition parameters and the mapping relationship between fatigue life includes: Collect historical optimal working condition parameters and their corresponding baseline fatigue life, and calculate the sensitivity index of each surface condition parameter to fatigue life with reference to the historical optimal working condition parameters. Based on the sensitivity index, a fatigue life prediction expression containing an exponential term and an exponential function term is established. Real-time surface condition parameters are input into the fatigue life prediction expression to calculate the fatigue life of the component. By comparing the calculated fatigue life with the design requirements, the pre-calibration parameters for the first-cycle impact response are determined.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention achieves a successful organic unity of ultra-low roughness, high steady-state gradient compressive stress, and high uniform surface energy on the surface of ultra-high strength steel through full-chain synergistic regulation. This avoids the surface damage and uncontrollable state problems caused by traditional mechanical strengthening methods such as shot peening, and also overcomes the bottleneck of heat treatment methods that cannot simultaneously consider stress amplitude, gradient, and thermal stability. The constructed residual stress field no longer depends on external plastic deformation, but is generated in situ by a self-generated thermo-mechanical coupling process induced by laser irradiation, and is solidified into a thermodynamic metastable state through subsequent multi-field synergistic effects, significantly improving its retention capability under temperature change conditions during service. At the same time, through molecular-scale directional reconstruction of the surface energy field, the anisotropy of surface free energy is eliminated, and the preferential adsorption and electrochemical activation tendency of the environmental medium in the stress concentration area is suppressed. This allows the component to exhibit consistent and predictable energy dissipation behavior when subjected to the first impact load, fundamentally improving fatigue performance stability and long-term service reliability. Attached Figure Description

[0016] Figure 1 This is a flowchart illustrating the preparation method of the ultra-high strength steel component with fatigue impact resistance according to the present invention. Detailed Implementation

[0017] The following description is intended to disclose the invention and enable those skilled in the art to implement it. The preferred embodiments described below are merely examples, and other obvious variations will occur to those skilled in the art.

[0018] like Figure 1 As shown, this invention proposes a method for preparing ultra-high strength steel components with fatigue impact resistance. Through synergistic control of the entire chain, it successfully achieves an organic unity of ultra-low roughness, high steady-state gradient compressive stress, and high uniform surface energy on the surface of ultra-high strength steel. This avoids the surface damage and uncontrollable state problems caused by traditional mechanical strengthening methods such as shot peening, and also overcomes the bottleneck of heat treatment methods that cannot simultaneously consider stress amplitude, gradient, and thermal stability. The specific steps include the following: The process of obtaining molten steel through vacuum induction melting and dynamic compositional control includes: implementing multi-stage pressure gradient degassing and dynamic oxygen potential control in a vacuum induction furnace to obtain a low-oxygen melt; performing directional migration enrichment of sulfur and phosphorus elements and slag-metal phase distribution control on the low-oxygen melt; adding microalloying elements to the melt after sulfur and phosphorus element reduction to achieve time-sequential addition and solute redistribution equalization based on solidification path constraints; and preparing a billet with a central density of ≥99.86% by directional solidification of the melt after microalloying element addition under multi-scale temperature field synergy.

[0019] Controlled atmosphere electroslag remelting of molten steel and gradient refinement of solidification structure are used to prepare remelted ingots. Specifically, this includes: using the billet as a consumable electrode, implementing low-fluorine slag system matching and dynamic compensation control of slag resistivity; under stable slag pool conditions, regulating the molten pool morphology and enhancing interface stability under dual-frequency electromagnetic field coupling; implementing zoned control of solidification structure under radial gradient cooling regime according to the stable molten pool morphology; and performing high-temperature homogenization and grain boundary phase dissolution kinetic control on the remelted ingot with solidification structure control to obtain a homogenized remelted ingot with a grain boundary phase area fraction ≤0.023%.

[0020] The remelted ingot is used as the billet for thermo-coupling net near-net forming and three-dimensional rheological homogenization to obtain a forging. Specifically, this includes: multi-stage gradient heating of the homogenized remelted ingot while identifying the dynamic width of the austenite phase region; when the heated billet is in the single-phase austenite region, closed-die forging path planning and metal streamline topology reconstruction are implemented; for the forging after metal streamline reconstruction, triaxial strain tensor equalization and dynamic recrystallization volume fraction control are performed; and a post-forging controlled cooling path is implemented for the forging after dynamic recrystallization, while suppressing the precipitation of proeutectoid phase, finally obtaining a net near-net-net forming forging with a grain size of 18.3 μm and no proeutectoid phase.

[0021] To achieve a surface roughness Ra≤0.08μm, a surface roughness Ra≤0.08μm was obtained by performing a combined surface roughness cutting-polishing process and quantitative stripping of the subsurface damage layer on forgings. Specifically, this involved: constructing a stiffness-matched high-speed cutting parameter domain for near-net-shape forgings while implementing closed-loop suppression of cutting forces; configuring a diamond micro-powder particle size gradient distribution on the cut surface and designing an elastic polishing pressure field; enhancing the electrochemical dissolution selectivity in electrolytic-mechanical composite polishing for the polished surface; and performing ultrasonic-assisted nanofluid final polishing on the electrolytically treated surface to promote surface energy state homogenization, ultimately obtaining a surface roughness Ra≤1.2μm with a subsurface plastic deformation layer depth and a surface energy ≤28.1mN / m.

[0022] Based on a light-quantized surface, local laser surface treatment and in-situ construction of a gradient compressive stress field are performed to form a surface layer with a residual compressive stress peak value ≥ -860MPa. Specifically, this includes: analyzing the absorption characteristics of the light-quantized surface, calculating the spatiotemporal distribution of heat flux density, and determining the multi-spot time-overlapping scanning path; calculating the martensite nucleation density based on the temperature field after thermal accumulation, and determining the austenitizing-self-tempering two-stage phase transformation path; calculating the axial residual stress distribution based on the microstructure characteristics after the phase transformation, and determining the laser-induced plastic compression parameters; combining the stress distribution gradient, calculating the stress relaxation rate, and determining the stress gradient cutoff and interface stress relaxation suppression process, ultimately obtaining a compressive stress field without stress reversal.

[0023] In-situ solidification of surface stress state and elimination of thermo-mechanical history memory are carried out on the surface layer to establish a thermodynamic metastable stress field. Specifically, this includes: implementing a dual-temperature zone stepped thermal cycling regime for the compressive stress field, designing a lattice distortion relaxation path, and synchronously applying a micro-compressive stress field to the surface after lattice distortion relaxation during the thermal cycling heat preservation stage to achieve interface stress redistribution; for the forging with completed stress redistribution, martensitic carbon partition stabilization under cooling path constraints is implemented; for the forging with completed carbon partition, the thermodynamic metastable state of the stress field is verified, the history memory effect is eliminated, and a thermodynamic metastable stress field with stress fluctuation amplitude ≤0.12% is obtained.

[0024] A hydrophobic surface with surface energy isotropic ≥0.97 is generated by reconstructing the surface energy field and directionally arranging the adsorption layer on a thermodynamic metastable stress field before service. Specifically, this includes: performing hydroxylation pretreatment on the thermodynamic metastable stress field surface and calibrating the density of active sites; performing directional grafting of bifunctional silanes on the hydroxylated surface to strengthen Si-O-Fe bonding; performing epoxy end-group closed-ring crosslinking on the silane-grafted surface to achieve surface energy isotropicization; and constructing nanoscale hydrophobic channels on the surface energy homogenized component to form a capillary blocking effect, thereby obtaining a hydrophobic surface with an average pore size of 24 nm and no capillary condensation at a relative humidity of 60%.

[0025] Based on hydrophobic surfaces, the system performs full-cycle service state mapping and fatigue impact response pre-calibration, outputting ultra-high-strength steel components with traceable service states. Specifically, this includes: analyzing the state parameters of the hydrophobic surface, calculating the mapping relationship between surface state parameters and fatigue life, calculating the plastic dissipation work in the first cycle based on the fatigue life mapping relationship, and determining the surface state fine-tuning parameters; combining the fine-tuned surface state, calculating the sensitivity of parameter drift to fatigue life, setting an early warning threshold, and calculating the coefficient of variation of the equivalent impact response multiple times for components with set early warning thresholds to verify the consistency of the full-cycle response.

[0026] The calculation of the mapping relationship between surface condition parameters and fatigue life includes: collecting historical best working condition parameters and their corresponding benchmark fatigue life; referring to the historical best working condition parameters, calculating the sensitivity index of each surface condition parameter to fatigue life; based on the sensitivity index, establishing a fatigue life prediction expression containing an exponential term and an exponential function term; inputting the real-time surface condition parameters into the fatigue life prediction expression to calculate the component fatigue life; comparing the calculated fatigue life with the design requirements to determine the pre-calibration parameters for the first-cycle impact response.

[0027] The following will describe in detail the specific steps of the above-mentioned method for preparing an ultra-high strength steel component with fatigue impact resistance, with reference to specific embodiments, including: Step 1: High-purity vacuum induction melting and dynamic synergistic control of composition This step aims to obtain liquid steel with highly uniform chemical composition and deep suppression of impurity elements, laying the material foundation for subsequent refinement of solidification structure and improvement of grain boundary cleanliness.

[0028] Step 1.1: Multi-stage pressure gradient degassing and dynamic oxygen potential control in a vacuum induction furnace Complete the proportions The raw materials are loaded into a graphite crucible and evacuated to a vacuum level. After Pa, the pressure was increased to 0.35 MPa argon atmosphere in three stages at a rate of 0.8 MPa / min; high-frequency induction heating was started simultaneously at a heating rate of 120 ℃ / min. When the temperature reached 1520 ℃, argon gas was sprayed onto the surface of the molten pool. The composite deoxidizing powder was injected at a flow rate of 3.6 kg / min for 110 s. After injection, the argon positive pressure was maintained at 0.35 MPa and the temperature at 1580 ℃ for 180 s, causing the dissolved oxygen mass fraction in the molten pool to decrease from an initial 0.0042% to 0.00017%. The dynamic oxygen potential environment established in this process can drive the interfacial reaction between O and Al and Si to generate low-melting-point inclusions, which are then carried to the slag layer by argon bubbles. The oxygen potential control here is not statically set but relies on the molten pool temperature. (K) and dissolved oxygen activity in the melt The thermodynamic coupling relationship between them is expressed by the following equilibrium expression: In the formula, The activity of oxygen in the melt is dimensionless. This is absolute temperature, expressed in Kelvin (K). The formula originates from... Standard Gibbs free energy change The derivation of the van der Hoff equation is applicable to the temperature range of 1520–1600 °C. When K is reached, the calculation yields... ,correspond The measured dissolved oxygen activity was 0.00017 (i.e., The discrepancy indicates that the presence of strong deoxidizing elements such as Al and Si in the actual system significantly reduces the effective oxygen activity. Therefore, the role of this formula here is to provide a reference for the theoretical lower limit of oxygen potential, guiding the timing of injection termination: when the measured oxygen content remains consistently below a certain level for 30 seconds... When the yield reaches wt.%, it indicates that the deoxidation reaction is approaching a kinetic plateau, and the injection should be stopped to avoid excessive reduction that could lead to abnormal fluctuations in the yields of Al and Si.

[0029] Step 1.2: Targeted migration and enrichment of sulfur and phosphorus elements and regulation of slag-metal phase distribution Based on the low-oxygen melt obtained in step 1.1, add preheated to 850 °C to the molten pool. The ternary refining slag, with a total slag content of 5.3% of the molten steel mass, of which... The slag composition was 42 wt.%, CaO 48 wt.%, and MgO 10 wt.%. After the slag layer formed, vertical mechanical vibration with a frequency of 42 Hz and an amplitude of 1.8 mm was applied for 210 s. During the vibration, the apparent viscosity of the molten pool decreased from an initial 8.7 dPa·s to 5.2 dPa·s, and the slag-steel interface renewal rate increased by 2.3 times. This vibration field enhanced interfacial mass transfer, promoting the accelerated migration of S and P atoms into the slag phase driven by the concentration gradient. Its migration flux... It obeys the modified form of Fick's second law: In the formula, This represents the interfacial migration flux of sulfur, expressed in mol / (m²·s). Let be the diffusion coefficient of sulfur in molten steel, and take a value of . m² / s (1580 ℃); This refers to the mass concentration of sulfur in molten steel, expressed in wt.%. The coordinates are perpendicular to the slag-steel interface, in meters. The molar gas constant is 8.314 J / (mol·K). The melt temperature (K); The partial molar Gibbs free energy of sulfur in the slag phase, and its spatial gradient This reflects the driving force of the slag phase chemical potential gradient on migration. Due to... Significantly reduces the degree of polymerization in the slag phase and increases Activity, thereby increasing ( Its solubility in slag makes The concentration of sulfur in the molten steel was increased from 12% to 47% in the conventional slag system. Ultimately, the sulfur content in the molten steel decreased from the initial 0.0021 wt.% to 0.00013 wt.%, and the phosphorus content decreased from 0.018 wt.% to 0.00046 wt.%, both of which entered the ultra-low impurity window.

[0030] Step 1.3: Sequential addition of microalloying elements and solute redistribution equalization based on solidification path constraints After slag washing in step 1.2, the melt is cooled to 1545 ℃ and held for 90 s. At this point, electromagnetic stirring is activated with a magnetic field strength of 18 mT and a rotation frequency of 3.2 Hz to generate annular eddies in the molten pool. Subsequently, microalloying components are added in three batches according to a time sequence: the first batch includes a V–C master alloy (78 wt.% V, 19 wt.% C), added at 0.124% of the molten steel mass, and electromagnetic stirring continues for 60 s; the second batch includes an Nb–Fe alloy (65 wt.% Nb), added at 0.037%, and stirred for 45 s; the third batch includes… A gas (99.999% purity) was introduced into the porous plug brick at the bottom of the molten pool at a flow rate of 0.23 L / min for 150 s, allowing nitrogen to dissolve in the melt in an atomic state. This timing design is based on the solidus temperature. The relative precipitation initiation temperatures of carbides and nitrides of each element are as follows: VC begins precipitation at 1420 ℃, NbC at 1360 ℃, and VN above 1480 ℃; while the solidus line of molten steel is located at 1415 ℃, therefore V preferentially precipitates as a carbide, Nb participates in grain boundary pinning at a later stage, and N remains in solid solution throughout to expand the austenite phase region. This arrangement ensures that the three do not compete for nucleation in the same temperature range, avoiding the formation of coarse composite inclusions. The solute redistribution behavior can be described by an extended version of the lever law: In the formula, For the first in the liquid phase Instantaneous concentration of a solute (e.g., V, Nb, N), expressed in wt.%% This represents the initial total concentration of the solute; For the first The effective partition coefficient of an element is defined as the ratio of its concentration in the solid phase to its concentration in the liquid phase. For V in... middle Nb is 0.21, and N is 0.032; This represents the volume fraction of the solidified solid phase. When When 15% solid phase is generated, the calculated residual V concentration in the liquid phase is 92.3% of the initial value, Nb is 89.1%, and N is 98.1%, indicating that nitrogen is least likely to be displaced to the liquid front, thus making it more conducive to achieving uniform solid solution between dendrites and providing support for the subsequent stability of austenite grains.

[0031] Step 1.4: Preparation of directional solidification of the initial billet and central densification under the synergistic effect of multi-scale temperature fields The homogeneous melt obtained in step 1.3 is poured into a copper mold preheated to 420 °C. The inner cavity dimensions of the mold are as follows: 850 mm, pouring temperature controlled at 1512 ℃, superheat The temperature was set at ℃. Immediately after casting, the three-stage water cooling system on the outer wall of the mold was activated: the water flow rate was 12.4 m / s in the first zone (0–200 mm height), 9.7 m / s in the second zone (200–550 mm), and 6.3 m / s in the third zone (550–850 mm). Simultaneously, a static pressure of 0.18 MPa was applied to the top of the ingot, continuing for 320 s after complete solidification. This pressure field, combined with the non-uniform cooling field, created a temperature gradient from top to bottom along the ingot's axial direction. K / mm, and lateral gradient K / mm, which makes the solid-liquid interface propulsion speed mm / s, satisfying The critical stability criterion is 52 K·s / mm, which inhibits lateral branching of columnar crystals and promotes the proliferation of equiaxed crystal nuclei. The central densification effect is determined by the volume fraction of shrinkage cavities. Quantification: In the formula, The final cavity volume fraction is dimensionless. Apply static pressure to the top, in Pa; The pressure holding time is measured in seconds (s). For the solid-to-solid ratio, take 7850 kg / m³. To determine the latent heat of solidification of molten steel, take J / kg; These are empirical parameters, calibrated using historical ingot X-ray tomography scans. Substitute This means the central macroscopic porosity is less than 0.14%, meeting the internal integrity requirements for high-load-bearing component blanks. This step outputs a cylindrical initial blank with a diameter of 280 mm and a length of 850 mm, with a sulfur content of... wt.% P content wt.% central density The structure is fine and equiaxed. Grain size, average grain size This provides a base material with both uniform composition and microstructure for the electroslag remelting step two.

[0032] Step 2: Controlled Atmosphere Electroslag Remelting and Gradient Refinement of Solidification Structure This step uses the initial billet output from step one as a consumable electrode and performs electroslag remelting under an inert atmosphere. The goal is to further purify the molten steel, eliminate macro- and micro-segregation, refine the solidification structure, and construct an austenite grain size gradient distribution in the radial direction of the ingot, making the outer grains finer than the core, thus providing structural reserves for dynamic recrystallization during subsequent hot working.

[0033] Step 2.1: Low-fluorine slag system matching and dynamic compensation control of molten slag resistivity The rough blank obtained in step one is machined into 750 mm consumable electrode, surface roughness To avoid unstable arc initiation; slag material is adopted The ternary system, in which CaO accounts for 54.2 wt.%. It contains 38.6 wt.% MgO and 7.2 wt.% fluoride-free material; the slag is dried at 110 ℃ for 6 h before being loaded. The water-cooled crystallizer is 115 mm thick, and the slag layer thickness is controlled at 115 mm. After remelting starts, the initial current is set to 5800 A, the voltage to 32 V, and the slag pool resistivity is... The initial value is 0.87 As smelting proceeds, the slag... The temperature gradually rises due to the dissolution of Al in the electrode, leading to... The voltage rises when the voltage in the slag pool is detected to rise to 34.5 V (i.e., V), automatic compensation mechanism: evenly spread MgO micro powder (particle size) preheated to 200 ℃ onto the slag surface. The single addition amount was 0.37% of the slag weight, after which the voltage dropped to 32.8 V. This compensation behavior is based on the relationship between the slag phase resistivity and composition: In the formula, For reference resistivity, we take 0.62. ; The activation energy for ion migration in the slag phase is taken as 128 kJ / mol. Same as before; For the scum mole fraction; The coefficient of resistivity enhancement was experimentally calibrated to be 3.1. When When the value increases from the initial 0.32 to 0.35, calculate... It will be 0.87 This corresponds to a voltage increase of approximately 2.9 V, consistent with the measured value. V matches. After the addition of MgO, due to its high dielectric constant ( Weakening the Al–O bond weakens the ion migration barrier, making Effectively reduced to 116 kJ / mol, thereby inhibiting Continue to climb. This control ensures the power density of the slag pit. ( Fluctuation range of the cross-sectional area of ​​the slag pit To ensure the depth of the molten pool The solidification interface morphology was stabilized at 102–108 mm, providing a prerequisite for subsequent control of the solidification interface morphology.

[0034] Step 2.2: Molten pool morphology control and interface stability enhancement under dual-frequency electromagnetic field coupling After the slag pool stabilizes in step 2.1, two sets of independent electromagnetic coils are arranged on the outer wall of the crystallizer: a low-frequency coil ( Hz, peak current A) Generates an axial main magnetic field to drive the overall circulation of the molten pool; high-frequency coil ( This generates a radial perturbation magnetic field, which is superimposed on the main field. The combined magnetic field of the two induces a Lorentz force in the molten pool. ,in For induced current density, To synthesize the magnetic induction intensity. This force field creates a two-scale flow within the molten pool: large-scale circulation controls macroscopic heat transfer in the melt, while small-scale turbulence suppresses dendrite bridging. The amplitude of the free surface undulations of the molten pool. Constrained by the following formula: In the formula, The surface tension of molten steel is taken as 1.82 N / m; The density of molten steel is taken as 7150 kg / m³. It is the acceleration due to gravity; The duration of the electromagnetic oscillation; The capillary time constant is .when When the exponent term approaches 1, The steady-state value reached 0.048 mm, far lower than the 0.19 mm without a magnetic field. The low-frequency circulation reduced the radius of curvature at the solid-liquid interface. The thickness increased from 8.2 mm during natural solidification to 14.6 mm, while high-frequency perturbations increased the local wavelength at the interface. The diameter was reduced from 120 mm to 43 mm, and the two worked together to increase the Péclet number of the interface. It decreased from 28.6 to 16.1. medium diffusion coefficient, The solidification front is lower than the absolute stability threshold of 18.5, thus completely suppressing the interfacial cellular-dendritic transition and obtaining a straight solidification front.

[0035] Step 2.3: Solidification microstructure zoning control under radial gradient cooling regime The crystallizer water cooling system is divided into four sections according to height: The cooling intensity of each section is determined by the water flow rate. The characteristics are, in order: Meanwhile, 24 thermocouples are embedded in the inner wall of the crystallizer to provide real-time feedback on the wall temperature. When a certain segment Deviation from set value At ℃, the water flow rate in the corresponding section is automatically adjusted. m / s. This gradient cooling creates a temperature gradient from the surface to the interior across the cross-section of the ingot. : in radius mm (center) K / mm, mm is At a depth of 3.9 K / mm, The solidification rate is 5.6 K / mm at the surface (mm). and They are positively correlated, according to Frank's criterion: In the formula, Let be the slope of the liquidus line, and take . Let be the solute diffusion coefficient, and take . The Gibbs-Thomson coefficient is taken as... The radial temperature second derivative is derived from measured values. Insertion is worthwhile K / mm². Substitute K / mm, therefore mm / s; while mm The rate difference results in shorter growth time and higher nucleation rate for the outer grains, while the core exhibits the opposite, ultimately leading to a smaller outer grain size. Core radial gradient distribution, gradient ratio This provides an organizational basis for strain coordination in step three of the hot working process.

[0036] Step 2.4: High-temperature homogenization of remelted ingots and kinetic control of grain boundary phase dissolution The result obtained in step 2.3 The 720 mm remelted ingot was transferred to a box-type resistance furnace and heated to 1220 °C at a rate of 3.5 °C / min. After holding at this temperature for 14 h, it was slowly cooled to 980 °C at a rate of 1.2 °C / min, held at this temperature for another 8 h, and finally cooled to room temperature with the furnace. This two-stage holding process targets two types of grain boundary phases: one is MC-type carbides (mainly V and Nb), whose dissolution initiation temperature... ℃; another type is the Laves phase ( ), ℃. The first stage of holding at 1220℃ aims to dissolve the Laves phase, and its dissolution process follows an Arrhenius-type diffusion governing equation: In the formula, The radius of the Laves phase particles is in meters. The heat preservation time is expressed in seconds (s). For Fe in The diffusion coefficient at 1220 °C is [value missing]. ; For Laves Medium saturated solubility, take wt.%% The total measured Mo+Nb content in the matrix was 0.87 wt.%. Let be the thickness of the diffusion layer at the phase interface, taken as 2.4 nm. Substitute... (Typical size), m / s, 14 h later Thinning amount was 54 nm, residual rate However, the second stage of holding at 980 ℃ activates short-range diffusion, causing the residual Laves phase to spheroidize and further dissolve. Ultimately, the area fraction of the Laves phase in the ingot decreases from 0.19% in the remelted state to 0.023%, while the MC-type carbides are completely dissolved. The output of this step is... The 720 mm homogenized remelted ingot has a clear radial grain gradient, clean grain boundary phases, and a macrosegregation index (MPI). This provides a blank with both structural and compositional stability for the near-net-shape forming process in step three.

[0037] Step 3: Thermo-coupling net-close forming and three-dimensional rheological homogenization This step uses the homogenized remelted ingot output from step two as the billet. Through closed-die forging combined with a multi-directional loading path, it achieves continuous envelope of metal flow lines, balanced distribution of triaxial strain, and isotropic microstructure within the near-net-shape profile.

[0038] Step 3.1: Multi-segment gradient heating and dynamic width identification of austenite phase region Will A 720 mm remelted ingot was placed into a roller hearth continuous heating furnace, and the temperature was controlled in three zones along the axial direction: the front zone (0–200 mm) was heated to 1160 °C at a rate of 2.8 °C / min, the middle zone (200–520 mm) was heated to 1185 °C at a rate of 3.2 °C / min, and the rear zone (520–720 mm) was heated to 1170 °C at a rate of 2.5 °C / min; the holding time for each zone was 25 min, and the atmosphere inside the furnace was at dew point. A nitrogen-hydrogen mixture at ℃ ( This gradient heating is designed to match the original radial temperature conduction lag of the billet: the outer layer heats up quickly, while the core heats up slowly. By combining a slightly lower temperature at the front end, the highest temperature in the middle section, and a slightly lower temperature at the tail end, the axial temperature difference of the entire billet is reduced when it exits the furnace. ℃, radial temperature difference ℃. Width of the austenite phase region. Determined by the measured phase transition point: In the formula, for The temperature at which the phase completely dissolves, i.e. point; The eutectoid transformation initiation temperature, i.e. Point. According to Thermo-Calc calculations and DSC verification, this steel grade ℃, therefore ℃. However, actual forging needs to avoid the two-phase region, so the effective single-phase... District The temperature is ℃, and the width is 55 ℃. The upper limit of the heating process in this step is 1185 ℃, and the front end is 1160 ℃ to leave a safety margin. After exiting the furnace, the surface temperature was measured to be 1178 ℃ and the core temperature was 1169 ℃, which is completely in the single-phase region.

[0039] Step 3.2: Closed-loop forging path planning and metal streamline topology reconstruction The heated blank is placed in a 125000 kN servo hydraulic press, and the mold cavity is the final component outline. 380 mm (with flange), preset three-pass deformation: first pass reduction mm, flattened to strain rate Second round mm, upsetting to Third round Finally forged Three passes of air cooling to 1120℃ followed by forging are performed to avoid strain accumulation and softening. The evolution of metal streamlines is constrained by volume conservation and velocity boundary conditions. In the formula, The contact area before and after deformation; This represents the velocity vector of the metal particles on the corresponding surface; The normal direction is for the area micro-element. The first pass has a large abrupt change in cross-section. The streamlines diverge radially, exhibiting a radial pattern; the second pass cross-section gradually changes. The shape tends towards axisymmetry and streamline convergence; the third-pass mold cavity is complex, with a flange area... Due to lateral constraints, the streamlines bypass the boss to form a closed loop. Ultimately, the streamlines completely enclose the transition area between the flange root and the shoulder, without any cut-off, turbulence, or folds, thus meeting the requirements for continuity of the load-bearing path.

[0040] Step 3.3: Triaxial strain tensor equilibration and dynamic recrystallization volume fraction control In each deformation pass in step 3.2, the internal strain tensor of the billet is inverted in real time using an array of pressure sensors embedded in the mold (32 points / cavity). And based on the Levy–Mises equation, the stress and strain increments are correlated: In the formula, For the increase of the strain tensor; For equivalent variable increment; Equivalent stress; This is the deviatoric stress tensor. The measured cumulative equivalent strain over three passes is... , where the axial radial Zhou Xiang Triaxial strain ratio All values ​​are greater than the isotropic threshold of 0.3. This strain state drives dynamic recrystallization (DRX), and its volume fraction... Described by the Avrami equation: In the formula, The critical strain is set to 0.31. The peak strain is taken as 0.48; The Avrami exponent is set to 2.35. When... hour, That is, recrystallization is almost complete, and the volume fraction of remaining unrecrystallized grains is [missing information]. Recrystallization grain size Given by the Sellars relation:

[0041] In the formula, For the average strain rate, take The deformation temperature is taken as 1165 K. The calculation yields... Compared to the core grains in step two ( It is refined by 5.4 times, and the entire cross section is uniform with no gradient residue.

[0042] Step 3.4: Post-forging controlled cooling path and suppression of proeutectoid precipitation The forging was immediately placed into a controllable air-cooling table after demolding, with a wind speed of Wind temperature The initial cooling temperature was 1150 °C; when the temperature dropped to 950 °C, static air cooling was switched on; at 820 °C, the temperature was transferred to a 220 °C salt bath for isothermal cooling for 35 minutes, followed by air cooling. This path spans... The eutectoid region (834–910 ℃) took only 112 s, far shorter than the nucleation incubation period of proeutectoid ferrite. This completely suppresses the precipitation of bulk ferrite. The eutectoid reaction rate is characterized by the Johnson–Mehl equation: In the formula, This represents the volume fraction of the eutectoid phase. The rate constant is 850 °C. ; This is the kernel dimension index, taken as 2.0. When This means the amount of eutectoid phase formed is negligible. The output of this step is... mm net near-formed forgings, with continuous streamline envelope and grain size The absence of proeutectoid phases and three-dimensional strain equilibrium provide geometric accuracy and surface microstructure consistency assurance for the surface photolithography process in step four.

[0043] Step 4: Surface Light-Quantitative Cutting-Polishing Co-processing and Quantitative Removal of Subsurface Damage Layer This step uses the near-net-shape forging output from step three as the blank, and achieves surface roughness through a sequential combination of high-speed cutting with carbide tools and elastic polishing with diamond micropowder. Subsurface plastic deformation layer depth No microcracks or white residue remain. The surface profile accuracy of the forging obtained in step three is... ) and surface grain size ( This constitutes the physical constraints for setting cutting parameters and polishing pressure.

[0044] Step 4.1: Construction of stiffness-matched high-speed cutting parameter domain and closed-loop suppression of cutting force The forging is clamped in a high-rigidity horizontal machining center (overall machine static rigidity). TiAlN coated carbide inserts (grade K20, cutting edge blunt radius) are selected. ), main declination angle blade inclination angle The cutting parameters are set as follows: cutting speed Back-biting knife quantity mm, feed rate mm / r; cooling method is minimum quantity lubrication (MQL), oil mist flow rate is 0.028 L / h. This parameter combination meets the cutting stiffness matching criterion: In the formula, These are the dynamic stiffnesses of the tool system and the workpiece system, respectively. Its elastic modulus (forgings) GPa, cutting tool GPa). Substitute The right-hand side value is obtained. However, actual measurements This ensures that cutting vibration is effectively suppressed. Cutting force Data is collected in real time by a force measuring instrument. At N, the system automatically reduces the speed by 5 m / min until... N, closed-loop response time ms. Finally, the surface is obtained. Surface layer microhardness Compared to the matrix ( The 7.0% increase indicates the presence of shallow work hardening, but there is no white layer (no amorphous phase detected by XRD), providing a controllable initial damage state for polishing in step 4.2.

[0045] Step 4.2: Design of Diamond Micropowder Particle Size Gradient Distribution and Elastic Polishing Pressure Field The workpiece cut in step 4.1 is transferred into a CNC polishing machine. The fixture is a flexible airbag (inflation pressure 0.12 MPa), and the polishing disc is a polyurethane matrix (Shore hardness A75). The polishing fluid is a water-based suspension containing three types of diamond micron powder with different particle sizes. (45% of volume) The concentrations were all 8.5 g / L; polishing pressure The initial pressure is applied by the airbag pressure and the additional weight. Increase every 30 seconds Ultimately The total polishing time was 210 s. This gradient particle size design is based on a material removal rate model: In the formula, The instantaneous removal rate is expressed in nm / s. The material property coefficient is taken as 0.023; For the first Seed particle size volume fraction; For the corresponding particle size, the unit is... ; The particle size index was experimentally calibrated to 1.68. Substituting the data, we get... Therefore, initially The final velocity was 2.37 nm / s. The pressure gradient design ensures rapid shaping with large particles in the initial polishing stage, followed by fine finishing with small particles in the later stage, avoiding localized over-polishing. The polished surface... Subsurface plastic deformation layer depth It is still slightly higher than the target value, and further stripping is required in step 4.3.

[0046] Step 4.3: Enhanced electrochemical dissolution selectivity in electrolytic-mechanical composite polishing The polished part from step 4.2 is used as the anode and immersed in... Mixed electrolyte ( The cathode is a stainless steel plate with an electrode spacing of 12 mm; a pulsed current is applied with a peak density of... The duty cycle was 35%, the frequency was 850 Hz, the workpiece rotation speed was 65 r / min, and the electrolysis time was 140 s. This process utilizes the electrochemical activity gradient caused by the difference in dislocation density: the dislocation density in the plastic deformation layer... matrix The former has an anodic dissolution current density It is 4.8 times that of the latter. Dissolution depth Derived from Faraday's law: In the formula, The molar mass of steel is taken as 55.85 g / mol; The average current density is taken as 4.2 A / dm². Time, in seconds; For current efficiency, we take 0.73; Density: 7.85 g / cm³; For the average price level, we take 2.1; Let C be the Faraday constant (96485 C / mol). Substituting this into the equation gives... It just covers the upper part of the residual plastic layer, while the amount of matrix dissolved is only... This allows for selective stripping. The surface after electrolysis... , To achieve the goal.

[0047] Step 4.4: Ultrasonic-assisted nanofluid final polishing and surface energy state homogenization Place the processed part from step 4.3 into an ultrasonic cleaning tank at a working frequency of 42 kHz. The tank contains... Nanofluids ( Particle size 18 nm, concentration 2.4 wt.%. The viscosity was 2.1 mPa·s; the workpiece passed through the fluid layer at a linear velocity of 0.8 m / s, and the ultrasonic treatment time was 85 s; the fluid temperature was constant at 32 ℃. Local high pressure was generated when the ultrasonic cavitation bubble collapsed. MPa) and high temperature ( K), causing nanoparticles to impact the surface with kinetic energy, preferentially clearing subsurface residual dislocation entanglement regions. Surface energy The wetting angle is related to the Young–Dupré equation. : In the formula, For the surface tension of the liquid (this fluid) ); This represents the contact angle of water on the polished surface. Before polishing. The decrease of 45.0% indicates that the surface atomic arrangement tends towards a thermodynamically stable state, and the dangling bond density is significantly reduced. The final surface... It has no microcracks, providing a clean, low-defect, high-energy reference surface for the local laser surface treatment in step five.

[0048] Step 5: Local laser surface treatment and in-situ construction of gradient compressive stress field This step uses the optically irradiated forging output from step four as the substrate, and applies pulsed fiber laser irradiation to the key load-bearing areas (flange root fillet, shoulder transition zone, bolt hole edges), aiming to achieve a surface layer thickness of 50–200 mm. Peak residual compressive stress within depth MPa, stress gradient The surface obtained in step four has a stress field of GPa / mm and is free of microcracks and non-equilibrium metastable stress fields of fused layers. Subsurface damage depth Surface energy These factors together determine the laser energy absorption rate, the starting position of the heat-affected zone, and the critical temperature threshold for phase transition, constituting the lower boundary of the process window for this step.

[0049] Step 5.1: Multi-spot temporal overlapping scan path planning and spatiotemporal decoupling of heat flux density The forging is clamped on a five-axis linkage laser processing platform. The laser source has a wavelength of 1070 nm and a beam quality of [missing information]. Single-mode fiber laser; focal length of focusing optical system mm, focal spot diameter Set single pulse energy mJ, pulse width ns, repetition frequency kHz; the scanning method uses a three-spot array (center + symmetrically offset left and right). The three light spots move in the same direction according to the time difference. Sequential triggering creates a quasi-continuous heating zone with spatially misaligned but superimposed thermal responses. This design is based on an approximate solution to the heat conduction equation under short-pulse conditions:

[0050] In the formula, Instantaneous heat flux density per unit volume, in units of ; Radial and axial coordinates; The beam waist radius ( ); The thermal diffusivity of steel ( (1100 ℃); erfc is the complementary error function. When the three spots are arranged according to... When staggered, in At that depth, the arrival time of the thermal peak of the second light spot coincides with the moment when the temperature of the first light spot decays to 72% of its peak value, while the third light spot maintains the temperature plateau at that depth. ms, thus in 30–60 A quasi-steady-state heat accumulation is formed within the interval, avoiding micro-melting or vaporization caused by single pulses. Measured surface temperature rise curves show that this path minimizes the impact on the heat-affected zone (...). (℃) Depth from 82 of a single light spot Expanded to 156 And the peak temperature gradient is from K / m decreased to K / m provides a thermophysical basis for subsequent stress field gradient control.

[0051] Step 5.2: Regulation of the two-stage phase transformation path of austenitization-self-tempering and enhancement of martensite nucleation density Based on the thermal accumulation in step 5.1, control the laser scanning speed. m / s, causing the irradiated area to experience rapid heating ( K / s), short-time austenitization ( ℃) and ultrafast self-tempering (cooling rate) K / s, cooling from 950 ℃ to 420 ℃ takes only 4.1 ms. This path avoids the pearlite-bainite transformation region, forcing the austenite to be supercooled to Martensitic shear occurs below ℃. Martensitic nucleation density. Determined by both interfacial energy and strain energy: In the formula, The theoretical maximum number of nucleation points (take) ); austenite / martensite interface energy (taken as 25) ); The driving force of phase change per unit volume ( Take at point ); The critical radius for nucleation is calculated to be 1.7 nm. The localized plastic strain induced by laser shock (inferred from the surface energy reduction in step 4.4) ); Let the reference strain be 0.0012. Substituting this into the equation yields... Compared to conventional quenching ( 90 times higher, corresponding to an average thickness of martensitic laths nm provides fine-grained reinforcement support for subsequent compressive stress stability.

[0052] Step 5.3: Modeling the self-generation mechanism of laser-induced plastic compression and thermally induced residual stress After laser irradiation, the surface layer undergoes compressive plastic strain due to instantaneous expansion constrained by the adjacent cold substrate. Upon cooling, this strain is partially retained as residual compressive stress. Its distribution is derived from the thermo-elastic-plastic constitutive relation, with axial residual stress... satisfy: In the formula, The elastic modulus is 212 GPa. The coefficient of thermal expansion ( ,average value); For depth The final cooling temperature at the point; The ambient temperature is 25 ℃. This represents the plastic strain at this depth. Due to the movement of the laser heat source, It exhibits an asymmetrical distribution: Substituting... That is, the peak compressive stress is located 12–18 mm below the surface. At this location, in the martensite enrichment region (25–65) The spatial overlap forms a "strong phase + strong stress" synergistic load-bearing structure. This stress field does not depend on external mechanical loading, but is generated spontaneously by a purely thermo-mechanical coupling process, hence it is called a self-generated compressive stress field.

[0053] Step 5.4: Stress gradient cutoff and interfacial stress relaxation suppression To prevent abrupt changes in the compressive stress field along the depth direction from causing interfacial shear instability, immediately after step 5.3, a nitrogen static pressure of 0.32 MPa was applied to the irradiated area for 110 s, with the gas temperature maintained at 180 °C. This thermo-pressure field acts on the non-equilibrium structure after laser treatment, promoting the development of high dislocation density regions in the surface martensite (…). Dynamic recovery occurs, but recrystallization is inhibited, resulting in dislocation cell size... The initial 28 nm was coarsened to 45 nm, while the local stress concentration factor was reduced. The stress relaxation rate is described by the Nabarro–Herring creep equation: In the formula, For material constants (take ) Local stress; The stress index is 1.0. This is the grain size index (taken as 2.0); Let K be the absolute temperature (453 K). Substitute K into... MPa ,have to MPa / s, stress decay within 110 s is 15.4 MPa, far below the allowable fluctuation ( (MPa), indicating that relaxation is controllable. The gradient compressive stress field is finally obtained: The stress-free reversal provides a mechanical barrier to suppress fatigue crack initiation under service loads in step six.

[0054] Step Six: In-situ curing of surface stress state and elimination of thermo-mechanical history memory This step takes the forging with gradient compressive stress field output from step five as the object. By superimposing a low-amplitude cyclic thermal load with a synchronous micro-compressive stress field, the unsteady thermal history effect introduced by laser treatment is eliminated without changing the surface microstructure and stress amplitude, allowing the residual stress field to enter a thermodynamic metastable state. The result from step five... MPa Pa / mm and martensitic lath thickness of 42 nm together constitute the upper limit constraints on the thermal cycling amplitude and pressure load of this step.

[0055] Step 6.1: Design of Dual-Temperature Stepped Thermal Cycling Mechanism and Lattice Distortion Relaxation Path The forging is placed in a vacuum heat treatment furnace (vacuum level) The temperature is divided into two zones along the axial direction (Pa): the main stress zone (flange root and shoulder) is heated to 320 ℃, held for 45 min, and then air-cooled to 120 ℃; the non-stress zone (middle of the rod) is simultaneously heated to 180 ℃ and held for 60 min. The temperature difference between the two zones is... This temperature difference ensures that thermal stress forms a self-balancing circulation within the principal stress region, rather than being transmitted across regions. This temperature difference drives lattice distortion relaxation, the degree of which... Given by Boltzmann type relations: In the formula, The dislocation pinning energy barrier is set to 0.48 eV. The thermal activation energy is taken as 1.32 eV. It is the gas constant; Let K be the absolute temperature (593 K). Substituting this into the equation, we get... This means that 99.2% of the mobile dislocations have completed unpinning and reorganization, but the pinning points (such as VC precipitates) have not dissolved, so the macroscopic stress amplitude remains unchanged. X-ray diffraction line width analysis confirms this. The full width at half maximum (FWHM) of the phase (martensite) diffraction peak decreased from 0.84° to 0.71°, corresponding to micro-strain. The decrease from 0.0021 to 0.0014 indicates that the lattice-scale distortion was effectively released.

[0056] Step 6.2: Synchronous loading of micro-compressive stress field and redistribution of interfacial stress Twelve minutes before the end of step 6.1's heat preservation, a flexible silicone gasket with a pressure of 0.18 MPa is applied to the surface of the main stress zone. The pressure direction is perpendicular to the surface, the gasket has a Shore hardness of A45, and the deformation is controlled to be 0.13 mm. This pressure, combined with the thermal expansion at 320 °C, results in... Additional compressive strain is generated within the layer. This, combined with the existing thermo-induced plastic strain, causes the stress peak location to shift from... Micro-move to ,at the same time Stress at the point of MPa rose to MPa, eliminating the tendency of surface tensile stress. This redistribution process satisfies Hooke's law for linear superposition under small deformations: In the formula, The pressure penetration depth (taken as 22) (calibrated by the elastic modulus and contact stiffness of the gasket). Characterize the pressure decay with depth. Substitute ,have to MPa, compared with the initial The design strength is improved by 16 MPa compared to the previous design, validating the effectiveness of the design.

[0057] Step 6.3: Stabilization of Martensitic Carbon Partition under Cooling Path Constraints When air-cooling forgings at 320 ℃, control the cooling rate. The temperature was increased by ℃ / s, then transferred to a 120℃ constant temperature bath and held for 90 min at 180℃, followed by air cooling to room temperature. This process enriches carbon atoms in the martensite into the retained austenite under metastable conditions, thus improving its stability. Carbon distribution ratio Defined as: In the formula, The original residual austenite carbon content (0.31 wt.%). The original martensite carbon content is 0.28 wt.%. This corresponds to the final state value. Based on the CCT plot and diffusion simulation, At ℃ / s, The value reached 0.73, which is higher than that of conventional air cooling ( This indicates that more carbon is retained in austenite, making it The temperature drops further, making it more difficult for the material to transform into martensite during service, thus inhibiting microcracks induced by phase transformation.

[0058] Step 6.4: Metastable verification of the stress field thermodynamics and erasure of historical memory The part treated in step 6.3 was heated again to 220 °C and held for 120 min, followed by air cooling. This process was repeated three times. Surface stress was measured after each cycle, and the results for the three cycles were as follows: fluctuation range This indicates that the stress field has broken free from thermal history dependence and entered a thermodynamic metastable state. This state is confirmed by the free energy minimization criterion: In the formula, For the system's Gibbs free energy; This is residual stress. When stress fluctuates... And if it does not increase for three consecutive times, then it is judged The condition is met when the system is at a local free energy minimum. The output of this step is a forging with stress history erased, whose compressive stress field no longer relaxes with slight fluctuations in service temperature, providing thermodynamic assurance for the fatigue performance stability during long-term service in step seven.

[0059] Step 7: Surface energy field reconstruction and adsorption layer orientation arrangement before service This step uses the metastable forging output from step six as the substrate, constructing a monolayer-thickness, uniformly oriented, and firmly bonded organosilane film on its surface. The goal is to reduce surface free energy anisotropy, inhibit preferential adsorption of water molecules in stress concentration areas, and block the initiation of electrochemical corrosion microcells. The result from step six... Together, they determine the adsorption configuration and coverage density of silane coupling agent molecules on the surface.

[0060] Step 7.1: Surface pretreatment for hydroxylation and active site density calibration Immerse the forging dilute nitric acid solution ( The steel was immersed in a solution of mol / L at room temperature for 190 s, followed by rinsing with deionized water and drying with nitrogen. This process generates a layer on the steel surface. Group, its surface density Measured by XPS The acid etching reaction follows the simplified form of the Langmuir adsorption isotherm: In the formula, The maximum hydroxyl coverage density (take) ); The adsorption equilibrium constant is (taken as...) ); activity( Substituting, we get... This density matches the measured values. This density provides sufficient uniform sites for subsequent silane molecule anchoring.

[0061] Step 7.2: Directional grafting of bifunctional silanes and Si–O–Fe bonding strengthening Prepare 1.2 vol.% A solution of glycidyl etheroxypropyltrimethoxysilane (KH-560) in ethanol was adjusted with 0.025 mol / L acetic acid. The pretreated forgings were immersed in the solution and reacted at a constant temperature of 65℃ for 135 min. After removal, they were rinsed with ethanol and dried under an infrared lamp. The methoxy group (in KH-560 molecule) The hydroxyl group hydrolyzes to silanol (–SiOH), which then condenses with Fe–OH to form Si–O–Fe covalent bonds; the epoxy ends are arranged outwards, reserving interfaces for subsequent functionalization. Grafting density Measured by ellipsometer ,occupy The bonding strength is 66%, which meets the theoretical limit (70%) for a single-layer close-packed structure. The bond strength is determined by the Si–O–Fe bond dissociation energy. The kJ / mol guarantee is far higher than that of van der Waals forces (10–50 kJ / mol), ensuring that it will not fall off during service.

[0062] Step 7.3: Epoxy end-group ring-closed crosslinking and surface energy isotropication The treated part from step 7.2 was placed in a 135 ℃ oven for 85 min. Under thermal action, the epoxy end of the KH-560 molecule underwent intramolecular ring closure, generating a stable six-membered ring ether structure, eliminating the polarity difference of the end groups. This reaction reduces the surface energy standard deviation. The concentration decreased from 3.2 mN / m before grafting to 0.8 mN / m, i.e. At any azimuth angle Upward fluctuation mN / m. Surface energy isotropicity. Defined as: In the formula, Standard deviation; The average surface energy is 27.4 mN / m. Substituting this into the equation, we get... This indicates that the surface energy is highly uniform and the water contact angle is [missing information]. Within the range of circumferential variation This greatly weakens the preferential adsorption tendency of water molecules in the stress concentration area.

[0063] Step 7.4: Construction of nanoscale hydrophobic channels and formation of capillary blocking effect Based on step 7.3, the forging is immersed in a solution containing... Nanoparticles The reaction was carried out in KH-560 ethanol solution at 65 °C for 45 min, followed by clotting at 135 °C for 60 min. Particles are embedded in the gaps of the silane network to form an average pore size Three-dimensional hydrophobic channels. When the ambient humidity... The critical radius at which water vapor condenses within capillary pores. Given by the Kelvin equation: In the formula, The relative humidity is 0.6. The surface tension of water is 72.8 mN / m. The molar volume of water ( ); Same as before. Substituting, we get... No capillary condensation occurs inside the pores, achieving passive blocking.

[0064] Step 8: Full-cycle service condition mapping and fatigue shock response pre-calibration This step takes the final-state forging output from step seven as the object, and establishes surface state parameters by associating it with the physical model through non-destructive characterization. ) and fatigue life The quantitative mapping relationship between them is established, and based on this, the component is calibrated before service to ensure that it exhibits optimal fatigue resistance under the first cycle of impact load. Step seven yields... The set of input variables that constitutes this mapping relationship.

[0065] Step 8.1: Construction of a multi-parameter coupled fatigue life prediction model Define fatigue life (In terms of the number of equivalent stress cycles) is a function of the four-dimensional vector of surface states: In the formula, The reference life (take) ); These are the historically optimal operating parameters; This is a dimensionless empirical index, calibrated using accelerated fatigue tests with 21 different process combinations. Substituting the current value, we get... It exceeds the design requirements ( ).

[0066] Step 8.2: Pre-calibration of the first-cycle impact response and fine-tuning of surface condition To ensure that the component withstands the rated impact load (peak stress) for the first time When the pressure reaches 0.08 MPa, the system enters its optimal energy consumption state, and the surface is fine-tuned: a static pressure of 0.08 MPa is applied to the flange root for 35 seconds, causing... Depend on MPa increased slightly to MPa (increased by 0.8%), at the same time From 0.073 Slightly increased to 0.075 (Increased by 2.7%). This fine-tuning resulted in... Depend on Upgraded to And the first cycle plastic dissipation work An increase of 11%, measured by a thermal imager of the surface temperature rise. The K value was increased from 0.42 K to 0.47 K, demonstrating enhanced energy dissipation capabilities.

[0067] Step 8.3: Initialization of the digital twin in service status and setting of parameter drift thresholds The final state parameters in step 8.2 ( Write the RFID chip into the component as the initial state of the digital twin. Set drift warning thresholds for each parameter. Any exceeding of the limit will trigger a maintenance prompt. This threshold is determined by the parameter pair. sensitivity Calculations show that, for example Therefore, a drift of 42 MPa corresponds to It decreased by 17.2%, reaching the lower limit of acceptable levels.

[0068] Step 8.4: Full-cycle response consistency verification and delivery status locking The calibrated component was subjected to three equivalent impact loads. Surface temperature rise was measured at 8 MPa pulse width (8 ms) with a 2-hour interval. Accumulated energy of acoustic emission Three results: coefficient of variation This indicates a high degree of consistency in the response. At this point, all parameters are locked, and the product is packaged and delivered. The output of this step is an ultra-high-strength steel component with predictable fatigue life, traceable state evolution, and calibrable first-cycle response, completing the closed loop from material preparation to service readiness.

[0069] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention. The scope of protection claimed by the appended claims and their equivalents is defined.

Claims

1. A method for preparing ultra-high strength steel components with fatigue impact resistance, characterized in that, include: Steel molten material is obtained by vacuum induction melting and dynamic synergistic control of composition. The steel molten material is then subjected to controlled atmosphere electroslag remelting and solidification microstructure gradient refinement to prepare remelted ingots. The remelted ingot is used as a billet to carry out thermo-coupled net near-forming and three-dimensional rheological homogenization to obtain a forging. The forging is then subjected to surface light-quantitative cutting-polishing co-processing and quantitative peeling of the subsurface damage layer to obtain a light-quantitative surface with a surface roughness Ra≤0.08μm. Local laser surface treatment and in-situ construction of gradient compressive stress field are carried out on the optically quantized surface to form a surface layer with residual compressive stress peak value ≥ -860MPa; The surface stress state is solidified in situ and the thermo-mechanical history memory is eliminated to establish a thermodynamic metastable stress field. On the thermodynamic metastable stress field, the surface energy field is reconstructed before service and the adsorption layer is oriented to generate a hydrophobic surface with surface energy isotropy ≥ 0.

97. Based on the hydrophobic surface, the full-cycle service status mapping and fatigue impact response pre-calibration are completed, and the output is an ultra-high strength steel component with traceable service status.

2. The method for preparing an ultra-high strength steel component with fatigue impact resistance according to claim 1, characterized in that, The vacuum induction melting and dynamic composition synergistic control include: A low-oxygen melt is obtained by implementing multi-stage pressure gradient degassing and dynamic oxygen potential control in a vacuum induction furnace. The low-oxygen melt was subjected to directional migration and enrichment of sulfur and phosphorus elements and regulation of slag-metal phase distribution. Adding microalloying elements to the melt after sulfur and phosphorus content reduction enables time-sequential addition and solute redistribution equalization based on solidification path constraints. The melt with added microalloying elements was subjected to directional solidification and central densification under multi-scale temperature field synergy to obtain a billet with a central density ≥99.86%.

3. The method for preparing an ultra-high strength steel component with fatigue impact resistance according to claim 2, characterized in that, The controlled atmosphere electroslag remelting and solidification microstructure refinement includes: The initial billet is used as a consumable electrode to implement low-fluorine slag system matching and dynamic compensation control of molten slag resistivity. Under stable slag pool conditions, the molten pool morphology is controlled and the interface stability is enhanced by dual-frequency electromagnetic field coupling. Based on the stable molten pool morphology, solidification structure zoning control is implemented under a radial gradient cooling regime; High-temperature homogenization and grain boundary phase dissolution kinetics control were applied to the remelted ingot with solidification structure control to obtain a homogenized remelted ingot with a grain boundary phase area fraction ≤0.023%.

4. The method for preparing an ultra-high strength steel component with fatigue impact resistance according to claim 3, characterized in that, The thermo-coupled net-close forming and three-dimensional rheological homogenization include: The homogenized remelted ingot is subjected to multi-stage gradient heating, while the dynamic width of the austenite phase region is identified; when the heated billet is in the single-phase austenite region, closed-loop die forging path planning and metal flow line topology reconstruction are implemented. For forgings after metal flowline reconstruction, triaxial strain tensor equilibration and dynamic recrystallization volume fraction control are performed. A controlled cooling path is implemented after forging for forgings that have undergone dynamic recrystallization, while suppressing the precipitation of proeutectoid phases, ultimately resulting in a net near-formed forging with a grain size of 18.3 μm and no proeutectoid phases.

5. The method for preparing an ultra-high strength steel component with fatigue impact resistance according to claim 4, characterized in that, The surface light-quantitative cutting-polishing co-processing and quantitative removal of the subsurface damage layer include: A stiffness-matching high-speed cutting parameter domain is constructed for the near-net-shape forging, while cutting force closed-loop suppression is implemented. On the surface obtained by cutting, a diamond micro-powder particle size gradient distribution is configured, and an elastic polishing pressure field is designed. Enhance the electrochemical dissolution selectivity in electrolytic-mechanical composite polishing for the polished surface; The electrolytically treated surface is then subjected to ultrasonic-assisted nanofluid final polishing, which promotes the homogenization of surface energy states, ultimately resulting in a light-quantized surface with a subsurface plastic deformation layer depth ≤1.2μm and a surface energy ≤28.1mN / m.

6. The method for preparing an ultra-high strength steel component with fatigue impact resistance according to claim 5, characterized in that, The localized laser surface treatment and in-situ construction of the gradient compressive stress field include: The absorption characteristics of the light-quantized surface are analyzed, the spatiotemporal distribution of heat flux density is calculated, and the multi-spot temporal overlapping scanning path is determined. Based on the temperature field after thermal accumulation, the martensite nucleation density is calculated to determine the austenitization-self-tempering two-stage phase transformation path; Based on the microstructure characteristics after phase transformation, the axial residual stress distribution is calculated, and the laser-induced plastic compression parameters are determined. By combining the stress distribution gradient, the stress relaxation rate is calculated, and the stress gradient cutoff and interface stress relaxation suppression process are determined, ultimately obtaining a compressive stress field without stress reversal.

7. The method for preparing an ultra-high strength steel component with fatigue impact resistance according to claim 6, characterized in that, The in-situ solidification and thermo-mechanical history memory elimination of the surface stress state include: A dual-temperature zone stepped thermal cycling regime is implemented on the compressive stress field, and a lattice distortion relaxation path is designed. During the thermal cycling heat preservation stage, a micro compressive stress field is synchronously applied to the surface after lattice distortion relaxation to achieve interface stress redistribution. For forgings with completed stress redistribution, martensitic carbon partition stabilization under cooling path constraints is implemented. For forgings with completed carbon partitioning, the thermodynamic metastable state of the stress field is verified, the historical memory effect is eliminated, and a thermodynamic metastable stress field with stress fluctuation amplitude ≤0.12% is obtained.

8. The method for preparing an ultra-high strength steel component with fatigue impact resistance according to claim 7, characterized in that, The pre-service surface energy field reconstruction and adsorption layer orientation arrangement include: The thermodynamic metastable stress field surface was pretreated with hydroxylation to determine the density of active sites; on the hydroxylated surface, bifunctional silane directional grafting was performed to strengthen Si-O-Fe bonding. Silane-grafted surfaces are subjected to epoxy end-group closed-ring crosslinking to achieve isotropic surface energy. By constructing nanoscale hydrophobic channels on a component with uniform surface energy, a capillary blocking effect is formed, resulting in a hydrophobic surface with an average pore size of 24 nm and no capillary condensation at a relative humidity of 60%.

9. The method for preparing an ultra-high strength steel component with fatigue impact resistance according to claim 8, characterized in that, The full-cycle service state mapping and fatigue shock response pre-calibration include: Analyze the state parameters of the hydrophobic surface, calculate the mapping relationship between the surface state parameters and fatigue life, calculate the plastic dissipation work in the first cycle based on the fatigue life mapping relationship, and determine the surface state fine-tuning parameters. Based on the surface condition after fine-tuning, the sensitivity of parameter drift to fatigue life is calculated, an early warning threshold is set, and for components with set early warning thresholds, the coefficient of variation of equivalent impact response is calculated multiple times to verify the consistency of response throughout the entire cycle.

10. The method for preparing an ultra-high strength steel component with fatigue impact resistance according to claim 9, characterized in that, The mapping relationship between the calculated surface condition parameters and fatigue life includes: Collect historical optimal working condition parameters and their corresponding baseline fatigue life, and calculate the sensitivity index of each surface condition parameter to fatigue life with reference to the historical optimal working condition parameters. Based on the sensitivity index, a fatigue life prediction expression containing an exponential term and an exponential function term is established. Real-time surface condition parameters are input into the fatigue life prediction expression to calculate the fatigue life of the component. By comparing the calculated fatigue life with the design requirements, the pre-calibration parameters for the first-cycle impact response are determined.

Images