Preparation method of modified iron-based alloy material with low expansion characteristic for precision mold

Abstract

The invention relates to the technical field of processing of modified iron-based alloy materials for precision molds, and discloses a preparation method of a modified iron-based alloy material for a precision mold with a low expansion characteristic, and the preparation method comprises the following steps: performing induction melting on a matrix, and introducing niobium and vanadium to construct a nucleation center; performing forging, spheroidizing annealing and austenitizing treatment on the blank; in the spraying cooling stage, the secondary change trend of the surface layer temperature along with the time change is monitored in real time so as to recognize the crystallization latent heat characteristic, then the spraying duration time is adjusted, and a martensite structure distributed discretely is induced and generated in the blank. And through dynamic balance adjustment of cold flow energy and latent heat release, a prestress field opposite to a thermal expansion vector is constructed, and the dimensional stability of the material under the high-hardness working condition is effectively improved.

Description

Technical Field

[0001] This invention relates to the field of processing technology of modified iron-based alloy materials for precision molds, and in particular to a method for preparing modified iron-based alloy materials for precision molds with low expansion characteristics. Background Technology

[0002] Current ultra-precision manufacturing places high demands on the dimensional stability of mold materials. In the preparation of precision molds, iron-nickel-cobalt based alloys with low expansion characteristics are used to counteract the thermal vibration of the crystal lattice by utilizing the magnetic volume effect. This mainstream approach maintains a small coefficient of thermal expansion in the room temperature range, providing a physical basis for precision forming. In actual service, molds need to have high hardness to maintain their service life. By introducing elements such as niobium and vanadium into the matrix to form dispersed carbides, the wear resistance of the material can be improved. However, the precipitation of carbides disrupts the symmetry of the matrix lattice, causing the magnetic volume effect to decay above 250°C. The continuous thermal cycling process induces microscopic phase transformation stress, resulting in irreversible dimensional shifts.

[0003] Existing technologies attempt to address stability issues by increasing cobalt content or extending the tempering cycle. However, simply altering the composition is insufficient to suppress the failure caused by the magnetic volume effect in the high-temperature region. In the preparation process of large-size billets, there is a physical phenomenon where martensitic transformation releases latent heat of crystallization. This latent heat alters the actual local cooling rate of the billet, causing the phase transformation process to deviate from the preset path. This thermal interference leads to uneven microstructure distribution and residual stress imbalance. Existing technologies mainly suffer from the following shortcomings: 1. Increased thermal expansion coefficient in the high-temperature region; 2. Large internal microstructure gradient in large components; 3. Dimensional zero-point shift under cyclic thermal loads.

[0004] Therefore, how to construct a strain field that antagonizes thermal expansion by regulating the cooling path and eliminate latent heat interference by utilizing a phase change feedback mechanism has become the technical problem to be solved by this invention. Summary of the Invention

[0005] To address the problems in the background technology, the present invention provides the following technical solution: a method for preparing a modified iron-based alloy material for precision molds with low expansion characteristics, comprising the following steps: Step S101: Vacuum induction melting is carried out according to the iron-nickel-cobalt matrix composition, and trace amounts of niobium and vanadium are introduced. By controlling the atomic ratio of niobium to vanadium in the range of 1.2 to 1.5, dispersed carbonitride nucleation centers are constructed in the melt to obtain modified iron-based alloy material. Step S102: The modified iron-based alloy material obtained by melting is subjected to hot forging and spheroidizing annealing to obtain a blank with an austenitic matrix structure. Step S103: Heat the blank to a temperature range of 850°C to 950°C and hold for 2 to 4 hours to perform complete austenitization. Step S104: The heated blank is moved into the cooling medium for spray cooling. The surface temperature of the blank is collected in real time using the temperature monitoring unit, and a sliding window monitoring model based on multiple continuous sampling points is established. The sliding window monitoring model is used to identify the secondary trend of the rate of change of surface temperature over time, so as to filter out the thermal interference signal generated by the flow of cooling medium and obtain pure temperature evolution characteristics. Step S105: Identify the characteristic peak value representing the release of latent heat of crystallization during the transformation from austenite to martensite in the secondary transformation trend. Adjust the spray duration of the cooling medium in a correlation with the intensity of the characteristic peak value. Utilize the cold energy captured by the spray cooling and the amount of latent heat of crystallization released to perform real-time energy balance adjustment. Induce the generation of discrete banded martensite structure inside the billet. The phase transformation prestress field formed by the martensite structure counteracts the lattice thermal expansion of the iron-nickel-cobalt matrix under heating conditions, and finally obtain a modified iron-based alloy material for precision molds with low expansion characteristics.

[0006] Preferably, the vacuum induction melting process in step S101 includes: establishing that the nickel content in the iron-nickel-cobalt matrix is ​​32% to 36%, the cobalt content is 4% to 6%, and the balance is iron and unavoidable impurities; adding ferroniobium and ferrovanadium at the end of the melting process, and utilizing the trace amounts of niobium and vanadium to agglomerate at the austenite grain boundaries.

[0007] Preferably, step S101 utilizes carbonitride nucleation centers to induce non-spontaneous nucleation during the cooling process, refining the martensite lath size to below 500 nm.

[0008] Preferably, in step S104, the sampling frequency for collecting surface temperature is not less than 50Hz, and the sliding window monitoring model is based on five consecutive sampling points.

[0009] Preferably, in step S105, the spraying duration is set to be positively correlated with the peak intensity of the secondary change trend. When the secondary change trend shows a positive deviation peak, the spraying duration is extended by adjusting the oil supply pressure of the spraying system.

[0010] Preferably, the adjustment logic for the spray duration W follows the following rules: ,in, The initial spraying time is the preset value, T is the surface temperature, t is the cooling time, and α is the preset proportional gain constant based on the thermophysical parameters of the modified iron-based alloy material.

[0011] Preferably, after step S105, step S106 is further included: performing multi-stage tempering on the cooled blank, with the tempering temperature set in the range of 450°C to 550°C, and performing isothermal dwell treatment between each tempering stage to induce solute atoms to segregate at the phase interface between martensite and austenite.

[0012] Preferably, the isothermal dwell time is 1 to 3 hours, which induces the generation of intermetallic compound nanoclusters that maintain a coherent relationship with the iron-nickel-cobalt matrix at the phase interface, serving as interface anchoring points to improve the shear resistance of the phase interface.

[0013] Preferably, the modified iron-based alloy material obtained has an average linear expansion coefficient lower than that in the temperature range of 25°C to 300°C. .

[0014] Preferably, in step S101, the unavoidable impurities have a phosphorus content of less than 0.005% and a sulfur content of less than 0.003%; the cooling medium in step S105 is quenching oil with pressure circulation regulation function.

[0015] The beneficial effects of this invention are: 1. In the preparation of modified iron-based alloy materials for precision molds, the dynamic counterbalance between the cold flow pulses and the release of latent heat of phase transformation inside the alloy during the segmented cooling operation induces a discrete banded phase transformation structure inside the billet. This distribution pattern constructs a prestress field at the lattice scale that is opposite to the thermal expansion vector of the matrix. The volume expansion generated by the martensitic transformation physically counteracts the lattice stretching of the matrix when heated, enabling the alloy to maintain high hardness while having an extremely low coefficient of thermal expansion, thus solving the problem of decreased dimensional stability of tool and die steel due to damage to lattice symmetry when heated.

[0016] 2. By utilizing the isothermal residence at subcritical temperature during multi-stage tempering to induce solute atom segregation, intermetallic compound nanoclusters that maintain a coherent relationship with the matrix are induced at the phase interface. These nanoclusters serve as interface anchoring points, locking the pre-compensated strain field by increasing the shear resistance of the phase interface, suppressing the thermally activated slip of dislocations under thermal shock cycles, avoiding zero-point drift of dimensions caused by stress relaxation, and enabling the mold to maintain high geometrical stability under long-term service conditions.

[0017] 3. Monitor the temperature rise characteristics of the billet surface during the spraying interval and use it as an endogenous feedback signal for the phase transformation progress to dynamically adjust the pulse width of the next cycle, so as to realize the orderly advancement of the phase transformation wavefront from the surface to the inside; eliminate the asynchronous phase transformation caused by the lag in heat conduction of large components, effectively reduce the residual stress concentration inside the material, and ensure the structural fidelity of the mold when performing complex and fine cavity processing. Attached Figure Description

[0018] Fig. 1 This is a flowchart of the process for preparing the modified iron-based alloy material and adjusting its latent heat feedback according to the present invention. Fig. 2 This is a comparison chart of the thermal expansion coefficient and dimensional offset of the material under different spraying parameters according to the present invention. Detailed Implementation

[0019] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the protection scope of the present invention.

[0020] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0021] Secondly, an embodiment or embodiment referred to herein refers to a specific feature, structure or characteristic that may be included in at least one implementation of the present invention. An embodiment appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0022] This invention is described in detail with reference to the schematic diagrams. When describing the embodiments of this invention, for ease of explanation, the cross-sectional views of the device structure will be partially enlarged without adhering to the general scale. Moreover, the schematic diagrams are only examples and should not limit the scope of protection of this invention. In addition, in actual manufacturing, the three-dimensional spatial dimensions of length, width and depth should be included.

[0023] Furthermore, in the description of this invention, it should be noted that the terms such as "upper," "lower," "inner," and "outer" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or component referred to has a specific orientation, or is constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention. In addition, the terms "first," "second," or "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0024] Unless otherwise explicitly specified and limited, the terms installation, connection, and linking in this invention should be interpreted broadly. For example, they can refer to fixed connection, detachable connection, or integrated connection; similarly, they can refer to mechanical connection, electrical connection, or direct connection, or indirect connection through an intermediate medium, or internal connection between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0025] This invention provides a method for preparing a modified iron-based alloy material for precision molds with low expansion characteristics. The method determines the composition system through vacuum melting and reconstructs the microstructure at the internal detail level through hot working and complete austenitization. The core of this method involves introducing an intermittent pulse quenching process based on latent heat of crystallization feedback during the cooling stage, followed by isothermal holding during multi-stage tempering to achieve coherent phase interface bonding. This establishes a precision mold material production system driven by composition, with a closed-loop process sequence and self-compensating microstructure. If uniform component distribution and extreme impurity control cannot be achieved during the melting stage of the precision mold material, non-uniform lattice stretching will occur when the matrix is ​​heated, thus disrupting the stability of the Invar effect. To address this technical challenge, in step S101, this invention performs vacuum induction melting according to the iron-nickel-cobalt matrix composition, determining that the nickel content in the matrix is ​​32% to 36%, the cobalt content is 4% to 6%, and the balance is iron. During the melting process, the phosphorus content and sulfur content in the impurities are controlled to be below 0.005% and low, respectively. At the end of the smelting process, 0.003% of ferroniobium and ferrovanadium are added at a mass ratio of 1.2 to 1.5 to introduce trace amounts of niobium and vanadium. Simultaneously, the chemical composition of the modified iron-based alloy material also includes 0.8% to 1.2% aluminum by mass percentage. This component melts together with the iron-nickel-cobalt matrix in a smelting environment with a vacuum degree better than 0.5 Pa to form a homogeneous solid solution. Aluminum atoms, as a key solute in the subsequent microstructure reconstruction stage, provide the material basis for atomic migration under the subcritical temperature isothermal residence procedure. If its content is below 0.8%, high-density pinning centers cannot be formed at the phase interface; if it is above 1.2%, it will induce an ordered transformation of the matrix, leading to material embrittlement. This limited content range ensures that the material maintains a low expansion vector while possessing sufficient yield strength. By generating dispersed carbonitride nucleation centers within the melt, the modified iron-based alloy material possesses microstructure refinement potential in its initial state, solving the problem of thermal expansion characteristic fluctuations caused by element segregation in traditional smelting processes.

[0026] To determine the key thermophysical parameters required for cooling control, a 1kg alloy sample was calibrated using differential scanning calorimetry (DSC) under a vacuum of 0.5Pa before batch processing. The measured latent heat of phase transformation per unit mass of this batch of material was 265,000 J / kg, and the specific heat capacity was 460 J / (kg·℃). Based on this, the calculated thermodynamic weighting parameter was 576.09℃. Within the aluminum content range of 0.8% to 1.2%, the cooling path driven by these parameters can ensure the formation of intermetallic compound nanoclusters with an areal density of no less than 1200 nanoclusters / μm² during the tempering stage, thus achieving the desired cooling performance. Physical pinning of phase interfaces; the microstructure of the blank before quenching directly affects the synergy of subsequent phase transformation. If the microstructure is coarse or the stress distribution is uneven, the expected discrete phase transformation band will not be generated. To address this issue, in steps S102 to S103, the modified iron-based alloy material is first subjected to hot forging and spheroidizing annealing to obtain a uniform austenitic matrix microstructure. The blank is then heated to the range of 850°C to 950°C and held for 2 to 4 hours to perform complete austenitization treatment, so that the carbide-forming elements are fully dissolved into the matrix, providing a microstructure basis for precise temperature control during the subsequent cooling process using latent heat feedback of phase transformation.

[0027] During the quenching process, large-sized mold blanks undergo martensitic phase transformation and release latent heat of crystallization. This heat source locally alters the undercooling, causing phase mismatch at the transformation front and inducing microstructure inhomogeneity. To address this phenomenon, this invention employs a latent heat response pulse width modulation method in step S104. A temperature monitoring unit collects the surface temperature T of the blank at a frequency of 50Hz. The system establishes a sliding window monitoring model based on five consecutive sampling points. The quadratic trend of the surface temperature change rate with time t is calculated. To filter out random interference caused by the flow of cooling medium and extract the characteristic peak of thermal rebound caused by the release of latent heat of crystallization, in the underlying algorithm implementation, the system collects the raw temperature signal every 20ms and pushes it into a step-type circular buffer. The sliding window monitoring model uses 5 fixed sampling points to form a calculation frame and sets the overlap rate between frames to 80%. Each additional sampling point triggers a second-order central difference calculation. To eliminate high-frequency spike artifacts at the 50Hz sampling frequency, a weighted average filter is performed on the 5 data points in the window before entering the secondary trend calculation. The weights are assigned from the far end to the near end of the time axis as 0.1, 0.1, 0.2, 0.2, and 0.4 respectively. The sliding window monitoring model acquires the surface temperature data of five consecutive sampling points and uses the second-order central difference operator to calculate the surface temperature. Over time The second trend of the rate of change In the non-phase change temperature region, the surface temperature is monitored to obtain the background baseline value of the natural cooling acceleration caused by physical heat transfer. Real-time collected secondary trend values ​​and background baseline values The difference ΔS exceeds When the latent heat of crystallization for the transformation of austenite to martensite begins to be released, the maximum value of ΔS is extracted as the characteristic peak value to correct the spray duration W, so that the cold energy capture rate and the latent heat release rate achieve energy balance in the time dimension, and induce the generation of banded martensite structure discontinuously distributed along the cold flow vector direction inside the billet.

[0028] To counteract the phase transformation stress and thermal expansion vector, in step S105, the system adjusts the spraying duration W of the cooling medium based on the identified peak intensity of the secondary change trend. The adjustment logic of the spraying duration W follows the formula: Where W is the spray duration after real-time adjustment. The preset initial spray duration is set to 1.5s in this embodiment; α is the thermophysical parameter proportional gain constant, with a value of 0.2; T is the surface temperature; t is the cooling time; and the initial spray duration is... The method for determining the proportional gain constant α is as follows: The geometric modulus M = V / A is calculated based on the billet volume V and the heat transfer area A. The coupling coefficient γ, which reflects the density and specific heat capacity characteristics of the modified iron-based alloy material, is selected to set the initial spraying duration. , The value is taken as 0.15 s / mm, which represents the latent heat of phase transformation per unit mass of the modified iron-based alloy material. and specific heat capacity Determine the thermodynamic weighting parameters The heat transfer efficiency factor κ is selected, and the proportional gain constant α is determined through the mapping relationship α=κ⋅ω. The value of κ is [value missing]. A quantitative mapping between metallurgical properties and cooling execution parameters was established to obtain martensitic structures refined to below 500 nm and exhibiting discrete banded distribution in blanks with different cross-sectional moduli. This enabled the phase transformation prestress field formed by the martensitic structure to achieve physical antagonism with the lattice thermal expansion of the iron-nickel-cobalt matrix under heating conditions.

[0029] When the sliding window monitoring model captures a positive bias peak, for example When the temperature reaches 4.8℃ / s², it indicates a rapid release of latent heat. The system extends the spraying duration and simultaneously increases the cold exchange rate by increasing the oil supply pressure. Through this energy balance regulation procedure, a discrete banded martensite structure is generated inside the billet, refining the martensite lath size to below 500nm. This phase transformation prestress field, driven by latent heat feedback, physically cancels out the thermal expansion vector of the iron-nickel-cobalt matrix, solving the dimensional stability problem caused by excessive internal microstructure gradient and asynchronous phase transformation in large-sized components. During control mapping, the controller will calculate... The obtained spray duration increment is converted into an analog voltage signal from 0V to 10V and output to the variable frequency oil pump. When the peak value of the secondary trend is detected to exceed 4.8℃ / s², the system increases the oil supply pressure at a ratio of 0.2V voltage increment for every 0.1℃ / s². At the same time, the high-level holding time of the electromagnetic spray valve is extended by 0.96s within the current 10s control cycle. By increasing the pressure, the cooling density per unit time is increased. Combined with the extension of the pulse width, this offsets the temperature rise fluctuation caused by the internal latent heat, ensuring that the cold flow capture rate is always more than 15% higher than the latent heat release rate.

[0030] The pre-compensated strain field is prone to atomic dislocation creep under the thermal cycling of the mold during service, causing the dimensional zero point to shift with increasing service time. To address this stability requirement, this invention performs multi-stage tempering after step S106. During the tempering process at 450℃ to 550℃, the system sets an isothermal residence period of 1 to 3 hours between each tempering stage. The residence procedure at this subcritical temperature causes aluminum atoms to undergo non-equilibrium segregation at the martensite-austenite phase interface. Within this segregation region, the interaction between aluminum atoms and matrix elements generates intermetallic compound nanoclusters with an L12 structure that maintain a coherent relationship with the iron-nickel-cobalt matrix. These nanoclusters act as interface anchoring points, locking the pre-compensated strain field by enhancing the shear resistance of the phase interface, thus realizing the transformation from phase transformation-induced to structural solidification and solving the problem of dimensional zero point drift in precision molds during long-term service. The resulting modified iron-based alloy material has an average linear expansion coefficient of less than 1.2 × 10⁻⁶ in the temperature range of 25℃ to 300℃. -6 / ℃, by capturing the latent heat characteristics of phase transformation in real time and adaptively counteracting the pulsed cold flow, the alignment of the martensite volume compensation in spatial distribution is ensured. Combined with the atomic-level energy pinning process at the interface, the material achieves structural stability under thermal shock conditions while maintaining high hardness and wear resistance. This preparation method, which utilizes dynamic thermal field balance to achieve highly stable dimensions, can be implemented by reconstructing the cooling logic of standardized vacuum heat treatment equipment and has engineering implementation value.

[0031] Example 1: In the scenario of fabricating cavity modules for large-size optical aspherical molds, the material needs to maintain nanoscale surface accuracy under high-temperature molding conditions of 300℃. Given that the working surface is subjected to alternating thermal shock, conventional Invar alloys cannot achieve both wear resistance and long-term dimensional stability through simple hardening. Using the aforementioned preparation method, vacuum induction melting is performed according to the iron-nickel-cobalt matrix ratio, with the atomic ratio of niobium to vanadium in the matrix being 1.35. This generates dispersed carbonitride nucleation centers within the melt, allowing for complete austenitization of the obtained blank. During the spray cooling stage, a single-point infrared thermometer is used to monitor the surface temperature T, and the collected temperature data is input into the controller to calculate the quadratic trend of the surface temperature change rate with time t. Where T is the surface temperature and t is time, the temperature at which the internal martensitic phase transformation is triggered and the latent heat of crystallization is released. The system generates a positive pulse peak value according to the formula. Adjust the spray duration W; where W is the spray duration. The initial spray duration is 1.5s, and α is the proportional gain constant with a value of 0.2.

[0032] When detected Reached 4.8 / s At peak intensity, the system adjusts the spraying duration from 1.5s to 2.46s, balancing the rate of cold flow energy capture and the intensity of latent heat release over time. This induces a prestress compensation band composed of martensitic laths refined to 450nm inside the blank. This procedure, which uses the latent heat signal during the cooling stage to invert the phase transformation progress and execute pulse adjustment, makes the volume expansion vector generated by the martensitic phase transformation antagonistic to the lattice stretching vector of the matrix when heated in physical space. Combined with the isothermal residence at 500℃ in the multi-stage tempering section, L12 structure intermetallic compound nanoclusters that maintain a coherent relationship with the matrix are generated at the phase interface. This enhances the shear resistance of the phase interface and locks the pre-compensated strain field, ensuring that the dimensional deviation of the modified iron-based alloy material after continuous thermal cycling is within 50nm, achieving a synergy of high hardness and low expansion physical properties.

[0033] Example 2: The experiment verified the low expansion stability of modified iron-based alloy materials under high-temperature molding conditions by establishing a high-performance mold steel thermal cycling test platform. The raw temperature data used in this experiment was directly collected from the physical test environment of the matching single-point infrared thermometer. Its temperature measurement resolution was 0.01℃ and the temperature control accuracy was within ±1℃. The sampling frequency was set to 100Hz. The consideration for determining the sampling frequency was to balance the capture accuracy of the latent heat release signal induced by martensitic phase transformation with the computing load of the controller. When the pulse width of the surface temperature fluctuation caused by the latent heat release inside the blank was 0.1s, the system determined the above parameters by logic that the sampling frequency was not less than twice the characteristic frequency to avoid aliasing. At the same time, Gaussian white noise with a signal-to-noise ratio of 20dB was injected into the temperature measurement circuit to simulate the electromagnetic background noise of the industrial site.

[0034] The experimental design was divided into three groups: experimental group A, control group B, and control group C. The experimental group employed the smelting, complete austenitization, latent heat response pulse quenching, and multi-stage tempering processes described in the preparation method. Control group A used a conventional cooling process with a constant flow rate. Control group B adjusted the atomic ratio of niobium to vanadium to 1.1. Control group C removed the isothermal residence step during tempering. All sample groups contained 34.2% nickel and 5.1% cobalt in their matrix chemical composition, with the balance being iron. After quenching was initiated, the system monitoring unit collected surface temperature T data, and the controller calculated the quadratic trend of the surface temperature change rate over time t. Where T is the surface temperature and t is time, when the phase transition front advances and causes the internal latent heat to be released instantaneously, the acquired signal is filtered to remove 20dB of background noise, and the temperature is detected. 4.78 was generated. / s The controller receives the characteristic wave peak and, according to the formula, performs the signal accordingly. Adjust the spray duration W; where W is the spray duration. The initial spray duration was set to 1.5 s in this experiment, and α was the proportional gain constant with a value of 0.2. The experimental group extended the spray duration to 2.456 s to offset the supercooling decay caused by the release of latent heat, and the induced martensite lath size was refined to about 432 nm. In contrast, the control group A produced a tissue with a size of 1.56 μm due to the lag in cooling response.

[0035] Verification of the gradient effect of the component ratio showed that when the atomic ratio of niobium to vanadium increased from 1.1 in the control group B to 1.35 in the experimental group, the average linear expansion coefficient of the material in the range of 25℃ to 300℃ increased from... Reduce to This confirms that the atomic ratio range determined in this invention compensates for the matrix expansion vector by generating dispersed carbonitrides. If the atomic ratio is increased to the performance saturation point of 1.6, the linear expansion coefficient of the material recovers to [a certain value]. Furthermore, the impact toughness decreased by 12.5%, confirming the physical basis for using the 1.2 to 1.5 range as the optimal working window. Dimensional stability tests showed that after 1000 thermal cycles at 25°C to 300°C, the control group, lacking the pinning effect of L12 structure intermetallic compound nanoclusters on the phase interface, experienced an irreversible dislocation creep deformation of 415.2 nm at its zero point. In contrast, the experimental group, through an isothermal dwell period at 500°C, drove aluminum atoms to segregate towards the interface and generate coherent nanoclusters, enhancing the phase interface shear resistance. The measured dimensional shift was 42.8 nm. Utilizing the latent heat of phase transformation for pulsed cooling regulation, combined with an interface locking process, enabled the modified iron-based alloy material to maintain a temperature below [value missing] under continuous thermal load conditions. The linear expansion coefficient achieves a balance between high hardness and high dimensional stability in precision mold materials.

[0036] Example 3: This example combines Figs. 1-2 The preparation method of a modified iron-based alloy material for precision molds with low expansion characteristics is described, such as... Fig. 1 As shown, step S101 involves vacuum induction melting according to the iron-nickel-cobalt matrix composition, introducing trace amounts of niobium and vanadium and controlling their atomic ratio within the range of 1.2 to 1.5 to construct dispersed carbonitride nucleation centers within the melt, thereby obtaining a modified iron-based alloy material. Step S102 involves hot forging and spheroidizing annealing of the obtained modified iron-based alloy material to obtain a billet with an austenitic matrix structure. Then, step S103 involves heating the billet to a temperature range of 850°C to 950°C and holding it for 2 to 4 hours to perform complete austenitization. Immediately following, step S104 involves... After heating, the blank is transferred into a cooling medium for spray cooling. The surface temperature is collected in real time using a temperature monitoring unit, and a sliding window monitoring model is established to identify the secondary trend of the surface temperature change rate, so as to filter out thermal interference signals and obtain pure temperature evolution characteristics. Finally, step S105 is executed to identify the characteristic peak value of latent heat of crystallization in the secondary trend. The spraying duration is adjusted according to the peak value intensity. By adjusting the energy balance between the cold energy and the latent heat release, a discrete banded martensitic structure is induced. The thermal expansion of the matrix lattice is offset by the phase transformation prestress field, and a modified iron-based alloy material with low expansion characteristics for precision molds is obtained.

[0037] like Fig. 2 As shown, the left vertical axis represents the coefficient of thermal expansion, with units of ×10⁻¹⁰. -6 / ℃, the right vertical axis represents the dimensional offset in nm, and the horizontal axis shows three different parameter settings. The legend indicates that the horizontal striped bars represent the coefficient of thermal expansion, and the diagonal striped bars represent the dimensional offset. In the first setting, when the initial spray duration is... When the time is 1.5s and the proportional gain constant α = 0.2, the coefficient of thermal expansion of the material is 1.11 × 10⁻⁶.-6 Near / ℃, the corresponding size offset is less than 44nm. In the second set of settings, when When α = 0.18 and t = 1.85s, the coefficient of thermal expansion decreases to 1.08 × 10⁻⁶. -6 / ℃, the size offset is between 46nm and 48nm, which is 47nm, while in the third set of settings when When α = 0.15 and s = 1.5s, both the coefficient of thermal expansion and the dimensional offset increase, reaching 1.15 × 10⁻⁶. -6 / ℃ and above 50nm.

[0038] Example 4: In the scenario of preparing a large-size precision thin film extrusion die with a deep cavity structure, the difference in geometric constraints between the surface and core of the blank causes a spatial mismatch in stress release during quenching. The blank suffers localized damage because the phase transition thermodynamics cannot be detected in real time. Using the preparation method described above, in step S101 (melting stage), an iron-nickel-cobalt matrix is ​​prepared with a nickel content of 34.5% and a cobalt content of 5.2%. By controlling the addition of ferroniobium and ferrovanadium, the areal density of carbonitride nucleation centers inside the blank is kept at 500 nuclei / mm². Up to 800 pieces / mm Within this range, the average spacing between nucleation centers is between 35 μm and 45 μm, and this spatial arrangement provides site constraints for the subsequent growth of martensite laths. During the spraying stage after complete austenitization, the control system initiates the latent heat response program, and the temperature monitoring unit collects the surface temperature at a sampling period Δt of 0.01 s. Where n is the sampling sequence number; to filter out random pulses generated by the impact of the cooling medium, the system processes 5 consecutive sampling points through a median filter and uses a second-order central difference operator to calculate the quadratic trend. The specific discretization calculation formula is as follows: ,in, This is the surface temperature collected at the current moment. The surface temperature at the previous moment. Δt represents the surface temperature at the previous two time points, and Δt represents the sampling period.

[0039] For thick-walled structures, the initial spraying duration The calibration procedure is performed as follows: the system determines the modulus M based on the volume-to-surface area ratio of the blank, measures the heat load per unit mass of the material near the phase transition point through thermal simulation experiments, and presets the modulus M using the principle of energy conservation. The value is 1.85s, and the proportional gain constant is determined using a heat exchange gain calibration test. It is 0.18s 3 / ℃; when the controller detects The temperature changed from negative to positive and exceeded 4.2℃ / s. 2When the threshold is reached, the program adjusts and extends the spraying time. This adjustment releases the volume expansion caused by the martensitic phase transformation within a specific time window, thereby physically neutralizing the tensile stress generated by the thermal contraction of the matrix lattice. When performing a multi-stage tempering process, the system controls the tempering furnace to heat to 500°C at a heating rate of 2°C / min and maintains an isothermal residence of 2.5h between each tempering stage. This method drives aluminum atoms to move towards the phase interface with a fixed diffusion flux, inducing the generation of intermetallic compound nanoclusters with an L12 structure whose average particle size distribution is in the range of 12nm to 18nm.

[0040] Example 5: In the process of determining the thermal response parameters of modified iron-based alloy materials, the system placed a sample block with a nickel content of 34.5% and a cobalt content of 5.2% in an induction furnace with a working vacuum of 0.45 Pa. After the sample block was heated to the austenitizing temperature of 900℃, an infrared thermometer monitored the evolution sequence of its temperature T with time t under natural cooling conditions. The controller used a first-order difference operator to calculate the thermal response time constant of this batch of materials. Time constant The formula for determining it is as follows: ;in, The current surface temperature. The surface temperature at the previous moment. For the current time, The previous moment The reference temperature for the furnace environment. As the thermal response time constant, the system will As a weighting operator, it is included in the secondary trend. In the calculation logic, it is used to correct the characteristic offset of the latent heat signal of phase change caused by the heat storage of the furnace body.

[0041] When the system processes mold blanks with different cross-sectional moduli, the controller calculates the geometric modulus based on the blank's volume V and heat exchange area A. Where V is the volume, A is the heat transfer area, and M is the geometric modulus. The system is based on the linear correlation formula. Determine the initial spraying duration ;in, The initial spraying time is γ, which is the coupling coefficient determined by the material density and specific heat capacity. In this embodiment, the value is 0.15 s / mm. This parameter establishes the initial cooling load per unit heat exchange area, so that the radial distribution deviation of the discrete martensite structure from the surface to the core of the blank is about 2.8%, and the dimensional zero-point offset of the modified iron-based alloy material in the 300℃ molding cycle is 45.2 nm.

[0042] Example 6: In a scenario where modified iron-based alloy mold blanks with dynamically changing surface roughness undergo batch heat treatment, gain calibration of the temperature signal is required during the complete austenitization stage to eliminate measurement deviations caused by changes in emissivity ϵ. Samples with representative surface conditions from the batch are selected and calibrated using contact thermocouples and an infrared thermometer. By adjusting the emissivity correction operator of the infrared thermometer, the deviation between the two readings is kept within ±0.5℃. The system monitors the secondary trend of surface temperature changes in the non-phase-change temperature range using a background reference value. ;in, To determine the natural cooling trend caused by physical heat transfer, the controller establishes a judgment procedure based on the difference between the real-time calculated trend value and the background reference value. Exceeding 1.2℃ / s 2 When the time is reached, it is determined to be the trigger point of internal martensitic phase transformation. This quantitative index is used to exclude the influence of environmental temperature field fluctuations on the extraction of phase transformation features.

[0043] When the system encounters changes in phase transformation energy density due to adjustments in the alloying degree of ferroniobium and ferrovanadium in a batch of raw materials, the operating point of the proportional gain constant α is determined by the following model reconstruction method, and the system extracts the latent heat of martensitic phase transformation per unit mass of the material for that batch. And based on the specific heat capacity of the matrix Calculate thermodynamic weight parameters ;in, The latent heat of phase transition per unit mass, ω is the specific heat capacity, and ω is the thermodynamic weighting parameter. The controller operates based on the mapping relationship. The parameter matrix is ​​updated; where α is the proportional gain constant and κ is the heat transfer efficiency factor. This method establishes a physical feedback between the metallurgical properties of the material and the amount of cooling medium applied, making the refinement of martensitic laths in the core of the blank more consistent with that on the surface. The resulting modified iron-based alloy material has an average linear expansion coefficient of 1.08 × 10⁻⁶ under a 300℃ molding environment. -6 / ℃ to 1.15×10 -6 Within the range of / ℃.

[0044] After the isothermal holding section of the multi-stage tempering process, the system controls the tempering furnace to perform furnace cooling at a cooling slope of no more than 5℃ / min until the billet temperature drops below 100℃, at which point the furnace door is opened for air cooling. The cooling path is designed to balance the segregation dynamics of solute atoms and the relaxation rate of interfacial stress release, thus avoiding incoherent precipitation of intermetallic compound nanoclusters with L12 structure from the phase interface due to transient thermal shock, thereby locking the pre-compensated strain field at the coherent interface of austenite and martensite.

[0045] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope defined by the present invention.

Claims

1. A method for preparing a modified iron-based alloy material for precision molds with low expansion characteristics, characterized in that, Includes the following steps: Step S101: Vacuum induction melting is carried out according to the iron-nickel-cobalt matrix composition, and trace amounts of niobium and vanadium are introduced. By controlling the atomic ratio of niobium to vanadium in the range of 1.2 to 1.5, dispersed carbonitride nucleation centers are constructed in the melt to obtain modified iron-based alloy material. Step S102: The modified iron-based alloy material obtained by melting is subjected to hot forging and spheroidizing annealing to obtain a blank with an austenitic matrix structure. Step S103: Heat the blank to a temperature range of 850°C to 950°C and hold for 2 to 4 hours to perform complete austenitization. Step S104: The heated blank is moved into the cooling medium for spray cooling. The surface temperature of the blank is collected in real time using the temperature monitoring unit, and a sliding window monitoring model based on multiple continuous sampling points is established. The sliding window monitoring model is used to identify the secondary trend of the rate of change of surface temperature over time, so as to filter out the thermal interference signal generated by the flow of cooling medium and obtain pure temperature evolution characteristics. Step S105: Identify the characteristic peak value representing the release of latent heat of crystallization during the transformation from austenite to martensite in the secondary transformation trend. Adjust the spray duration of the cooling medium in a correlation with the intensity of the characteristic peak value. Utilize the cold energy captured by the spray cooling and the amount of latent heat of crystallization released to perform real-time energy balance adjustment. Induce the generation of discrete banded martensite structure inside the billet. The phase transformation prestress field formed by the martensite structure counteracts the lattice thermal expansion of the iron-nickel-cobalt matrix under heating conditions, and finally obtain a modified iron-based alloy material for precision molds with low expansion characteristics.

2. The method for preparing a modified iron-based alloy material for precision molds with low expansion characteristics according to claim 1, characterized in that, The vacuum induction melting process in step S101 includes: establishing that the nickel content in the iron-nickel-cobalt matrix is ​​32% to 36%, the cobalt content is 4% to 6%, and the balance is iron and unavoidable impurities; adding ferroniobium and ferrovanadium at the end of the melting process, and utilizing the trace amounts of niobium and vanadium to agglomerate at the austenite grain boundaries.

3. The method for preparing a modified iron-based alloy material for precision molds with low expansion characteristics according to claim 2, characterized in that, Step S101 utilizes carbonitride nucleation centers to induce non-spontaneous nucleation during the cooling process.

4. The method for preparing a modified iron-based alloy material for precision molds with low expansion characteristics according to claim 1, characterized in that, In step S104, the sampling frequency for collecting surface temperature is not less than 50Hz, and the sliding window monitoring model is based on five consecutive sampling points.

5. The method for preparing a modified iron-based alloy material for precision molds with low expansion characteristics according to claim 1, characterized in that, In step S105, the spraying duration is set to be positively correlated with the peak intensity of the secondary change trend. When the secondary change trend shows a positive deviation peak, the spraying duration is extended by adjusting the oil supply pressure of the spraying system.

6. The method for preparing a modified iron-based alloy material for precision molds with low expansion characteristics according to claim 5, characterized in that, The adjustment logic for the spray duration W follows these rules: ,in, The initial spraying time is the preset value, T is the surface temperature, t is the cooling time, and α is the preset proportional gain constant based on the thermophysical parameters of the modified iron-based alloy material.

7. The method for preparing a modified iron-based alloy material for precision molds with low expansion characteristics according to claim 1, characterized in that, After step S105, step S106 is also included: the cooled blank is subjected to multi-stage tempering treatment, the tempering temperature is set in the range of 450°C to 550°C, and isothermal residence treatment is performed between each tempering stage to induce solute atoms to segregate at the phase interface between martensite and austenite.

8. The method for preparing a modified iron-based alloy material for precision molds with low expansion characteristics according to claim 7, characterized in that, The isothermal dwell time is 1 to 3 hours, which induces the formation of intermetallic compound nanoclusters that are coherent with the iron-nickel-cobalt matrix at the phase interface, serving as interface anchoring points to improve the shear resistance of the phase interface.

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