Performance testing method and testing device for high-strength steel non-isothermal liquid nitrogen cooling deformation

Abstract

The present application relates to a kind of high-strength steel non-isothermal liquid nitrogen cooling deformation performance test method and testing device.High-strength steel non-isothermal liquid nitrogen cooling deformation performance test method, it is cooled to sample using liquid nitrogen jet, follow-up cooling, and carry out non-isothermal tensile test.The performance test method includes the following steps: S1, preliminary preparation;S2, high temperature heating;S3, non-isothermal stretching.The present application can obtain high-temperature hot forming limit under the condition of liquid nitrogen rapid cooling and high strain rate, can accurately capture the yield strength mutation and forming limit inflection point of material under the condition of rapid cooling, realize accurate prediction of high-temperature non-isothermal forming of sheet metal, while two sub-experiments are verified, by from microstructure to explore the influence of liquid nitrogen rapid cooling and high strain, strain rate on the change of organization before hot stamping quenching and pressure holding process of high-temperature sheet, finally explore the influence law of liquid nitrogen injection process on sheet forming quality.

Description

Technical Field

[0001] This invention relates to the field of material mechanical property testing technology, and in particular to a performance testing method for non-isothermal liquid nitrogen cooling deformation of high-strength steel and a testing device for non-isothermal liquid nitrogen cooling deformation of high-strength steel. Background Technology

[0002] High-strength steel, with its high strength, high toughness, and fatigue resistance, is widely used in key fields such as automobiles and aerospace. Currently, the automotive industry focuses on producing high-strength, lightweight components, which involves high-temperature sheet metal processing. To this end, many scholars are keen to study the thermodynamic parameters and material properties of high-strength steel. However, current technologies often employ isothermal tensile testing: the steel is cooled to different temperatures according to the experimental design requirements, followed by tensile testing and data recording. In actual production, when forming using equipment such as roller hearth furnaces and hydraulic presses, static air cooling or water cooling techniques are used for the steel. In this case, the coordination between cooling rate and high strain rate must be considered; otherwise, the assessment of material properties and thermodynamic parameters during non-isothermal deformation at high temperatures will be inaccurate. For example, high-temperature hot forming limit diagrams are widely used to guide the hot stamping of metal sheets. These diagrams typically obtain the forming limit surface at a constant temperature, which differs from the continuous temperature change of the sheet metal during actual high-temperature hot stamping, making it difficult to accurately guide the hot stamping of metal sheets. Summary of the Invention

[0003] Therefore, it is necessary to provide a performance testing method and device for non-isothermal liquid nitrogen cooling deformation of high-strength steel, which addresses the problem that existing technologies do not consider the synergy between cooling rate and high strain rate, leading to inaccurate evaluation of material properties and thermodynamic parameters during non-isothermal deformation at high temperatures.

[0004] In a first aspect, this invention proposes a performance testing method for high-strength steel subjected to non-isothermal liquid nitrogen cooling deformation. This method employs a liquid nitrogen jet to rapidly cool the sample and then conducts a non-isothermal tensile test. The non-isothermal tensile test includes: a sub-experiment one investigating the properties of the sample before the martensitic transformation temperature Ms, which is used to obtain the forming limit of the sample under rapid cooling at high temperature; and a sub-experiment two investigating the properties of the sample after Ms.

[0005] Sub-experiment 1 includes the following steps:

[0006] S311, Set I initial temperatures T1, T2, ..., T I The heat-insulated sample was then air-cooled to the i-th initial temperature T. i i∈[1, I]. The sample is air-cooled to the i-th initial temperature T. i Then, the specimen was subjected to strain rate Perform tensioning; simultaneously, cool the specimen at an average cooling rate V. ij Cool the sample and break it before the sample temperature drops to Ms.

[0007] in, and V ij The relationship between them is satisfied:

[0008]

[0009] T f =T i -t i ·V ij

[0010] In the formula, t i T is the cooling time; A is the tensile limit strain in the austenitic holding state and isothermal state; T is the tensile limit strain. f The temperature at which the sample breaks is denoted as .

[0011] S312, Traverse T1, T2, ..., T I The parameters of the yield criterion during the tensile process are recorded; the recorded parameters are then input into the MK model to predict the forming limit of the specimen under high temperature and rapid cooling.

[0012] Sub-experiment 2 includes the following steps:

[0013] S321, The sample is air-cooled to the i-th initial temperature T. i Then, the specimen was subjected to strain rate Applying the dependent variable ε i Perform tensioning; while stretching, first cool the sample according to the average cooling rate V. ij Cool the sample until the temperature drops to the martensitic transformation temperature M. f The sample was then cooled to room temperature at the maximum cooling rate Vmax.

[0014] S322. Cut the strain concentration part of the specimen to obtain the experimental part; conduct a room temperature tensile test on the experimental part to obtain mechanical property indicators, and obtain the changes in the specimen microstructure in sub-experiment one based on the mechanical property indicators.

[0015] S323, Traverse T1, T2, ..., T I Obtain I initial temperatures T1, T2, ..., T I The mechanical properties of the sample were determined.

[0016] Secondly, the present invention also proposes a testing device for non-isothermal liquid nitrogen cooling deformation of high-strength steel, which uses the performance testing method for non-isothermal liquid nitrogen cooling deformation of high-strength steel in the first aspect. The testing device for non-isothermal liquid nitrogen cooling deformation of high-strength steel includes: a tensioning mechanism for stretching the sample, a monitoring module for monitoring the temperature and strain of the sample, a cutting mechanism for cutting the strain concentration portion of the sample, and a liquid nitrogen jetting mechanism for rapidly cooling the sample.

[0017] The liquid nitrogen jet mechanism includes: a liquid nitrogen tank, a solenoid valve, a nozzle, and a triaxial movement module; the outlet of the liquid nitrogen tank is connected to the nozzle; the nozzle is connected to the triaxial movement module; the solenoid valve is located at the outlet of the liquid nitrogen tank and is used to control the opening and closing of the liquid nitrogen tank; the triaxial movement module is used to controllably adjust the position between the nozzle and the sample, and to adjust the nozzle displacement and the sample cooling deformation area in non-isothermal tensile tests, so that the cooling deformation area and cooling effect of the sample can be kept constant by adjusting the up-down and back-forward displacement of the nozzle in non-isothermal tensile tests.

[0018] The beneficial effects of this invention are as follows:

[0019] This invention can obtain the high-temperature hot forming limit under rapid liquid nitrogen cooling and high strain rate conditions. It can accurately capture the abrupt change in yield strength and the inflection point of forming limit of materials under rapid cooling conditions, and realize accurate prediction of high-temperature non-isothermal forming of sheet metal. At the same time, two sub-experiments verify each other. By exploring the microstructure, the influence of rapid liquid nitrogen cooling and high strain and strain rate on the microstructure changes of high-temperature sheet metal before hot stamping quenching and holding process is explored. Finally, the influence law of liquid nitrogen spraying process on the forming quality of sheet metal is explored. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 A flowchart for a performance testing method of high-strength steel under non-isothermal liquid nitrogen cooling deformation;

[0022] Figure 2 This is a schematic diagram of the transient forming limit tensile test under high temperature liquid nitrogen in Experiment 1.

[0023] Figure 3 This is a tensile diagram illustrating the pre-strain effect under high-temperature liquid nitrogen in Experiment 2.

[0024] Figure 4A schematic diagram of the test device for non-isothermal liquid nitrogen cooling deformation of high-strength steel;

[0025] Figure 5 This is a schematic diagram of the three-axis motion module.

[0026] In the diagram: 1. Slide table; 2. Thermal imager; 3. Green light; 4. Flow valve; 5. Industrial camera; 6. Solenoid valve; 7. Liquid nitrogen tank; 8. Servo motor; 9. Coupling; 10. Synchronous belt; 11. Fixture; 12. Three-axis moving module; 121. Dovetail slide rail; 122. Lateral slide rail; 123. Base; 13. Ball screw; 14. Nozzle; 15. Tension sensor. Detailed Implementation

[0027] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] It should be noted that when a component is said to be "installed on" another component, it can be directly on the other component or it may be in a component that is centered on it. When a component is said to be "set on" another component, it can be directly set on the other component or it may also be in a component that is centered on it. When a component is said to be "fixed to" another component, it can be directly fixed to the other component or it may also be in a component that is centered on it.

[0029] It should be understood that the terms "length", "width", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., 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 application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0030] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the specification of this invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "or / and" as used herein includes any and all combinations of one or more of the associated listed items.

[0031] This invention achieves rapid cooling of high-strength steel by precisely controlling the pressure, flow rate, time, and position of the liquid nitrogen jet, utilizing the low-temperature properties of liquid nitrogen. Simultaneously, the strain, strain rate, and forming temperature range of the testing device are adjusted to control the non-isothermal tensile deformation process of the high-temperature high-strength steel during rapid cooling. This multi-parameter coupling method combines deformation control with the high-temperature rapid cooling process, thereby optimizing the non-isothermal forming performance of high-strength steel under liquid nitrogen spraying. By in-depth analysis of the tensile data of high-strength steel under different strains and strain rates within a specified temperature range under different cooling paths, the influence of different non-isothermal processes on the formed surface of high-strength steel can be comprehensively explored.

[0032] Specifically, please refer to Figure 1 This embodiment provides a performance testing method for high-strength steel undergoing non-isothermal liquid nitrogen cooling deformation. The method employs a liquid nitrogen jet to rapidly cool the sample and then conducts a non-isothermal tensile test. The performance testing method includes the following steps: S1, preliminary preparation; S2, high-temperature heating; S3, non-isothermal tensile testing. Each step will be explained in detail below:

[0033] S1. Preliminary preparations.

[0034] This step includes two sub-steps: strain measurement preparation and temperature measurement preparation.

[0035] The strain measurement preparation includes speckle fabrication and camera calibration. First, suitable speckles are created on the sample surface. Then, digital image correlation (DIC) is used to calculate displacement and strain by analyzing the grayscale information of the sample surface before and after deformation. The DIC used is a non-contact optical measurement method widely used to acquire deformation displacement and strain information of objects, especially suitable for measurements in high-temperature environments. During strain measurement, white high-temperature resistant paint is sprayed at room temperature, combined with green illumination and a corresponding wavelength filter to effectively suppress blackbody radiation interference and improve the grayscale of the material surface, ensuring high-precision identification and measurement of DIC below 1000℃.

[0036] Temperature measurement should be performed using either a thermocouple (contact) or a thermal imager (non-contact). If using a thermocouple, first sand the sample surface to ensure good contact between the thermocouple and the sample. Then, use a spot welding machine to firmly weld the thermocouple to the sample surface. Finally, cure the sample using a high-temperature resistant, highly adhesive, and thermally conductive ceramic adhesive.

[0037] S2, High-temperature heating.

[0038] To investigate the non-isothermal deformation process of cooled austenitic specimens, the experiment required ensuring that the initial deformed grains were austenitic. In this step, the specimens were uniformly heated in a resistance furnace at a heating rate of 15℃ / s to 920℃ and held for 5 minutes.

[0039] S3, Non-isothermal stretching.

[0040] This step aims to investigate the mechanical properties of fully austenitic material during its transformation to martensite under non-isothermal liquid nitrogen cooling conditions, with a focus on the changes in non-isothermal tensile properties. This is achieved by setting a specific temperature range: initial temperature T... i The martensitic initiation temperature Ms (approximately 420℃) and the martensitic complete transformation temperature M f (Approximately 200℃) Study on liquid nitrogen flow rate (cooling effect) V ij strain rate and dependent variable ε i The influence on the mechanical properties of the material. In this embodiment, the non-isothermal tensile test includes: Sub-Experiment 1, which investigates the properties of the specimen before the martensitic transformation temperature Ms, and is used to obtain the forming limit of the specimen under rapid cooling at high temperature; and Sub-Experiment 2, which investigates the properties of the specimen after Ms. Sub-Experiment 2 can also investigate the changes in the microstructure of the specimen caused by the process parameters in Sub-Experiment 1.

[0041] Sub-experiment one includes the following steps:

[0042] S311, Set I initial temperatures T1, T2, ..., T I The heat-insulated sample was then air-cooled to the i-th initial temperature T. i i∈[1, I]. The sample is air-cooled to the i-th initial temperature T. i Then, the specimen was subjected to strain rate Perform tensioning; simultaneously, cool the specimen at an average cooling rate V. ij Cool the sample and break it before the sample temperature drops to Ms.

[0043] Wherein, the initial temperatures T1, T2, ..., T I The determination method is based on the following formula:

[0044] T i =Ms+iΔT

[0045]

[0046] In the formula, ΔT represents the initial temperature interval and is an integer. Af is the complete austenite transformation temperature of the sample.

[0047] according to The differences can be categorized into different states: quasi-static strain rate, which ranges from 10... -5 ~10 -1 S -1 Dynamic strain rate, including medium strain rate: 1~10 2 S -1High strain rate: 10 2 ~10 4 S -1 Ultra-high strain rate: >10 4 S -1 . During the setup, V needs to be achieved by controlling the liquid nitrogen flow rate. ij The adjustment. V ij The determination method is based on the following formula:

[0048] V ij =V ij min+K ij ΔV

[0049]

[0050]

[0051] In the formula, V ij min is the minimum cooling rate before Ms is reached when the specimen breaks. K ij DT is a constant. ΔV is the cooling interval. i This is a preset temperature range. M represents different V values. ij The number of groups is set.

[0052] Determine V ij After that, and V ij The relationship between them is satisfied:

[0053]

[0054] T f =T i -t i ·V ij

[0055] In the formula, t i T is the cooling time. A is the tensile limit strain in the austenitic holding state and isothermal state. f This is the temperature at which the sample breaks. It should be noted that if T... f If the result is ≥Ms, then the data for that experiment should be recorded. Otherwise, the data is unreliable and should not be recorded.

[0056] S312, Traverse T1, T2, ..., T I The parameters of the yield criterion under high-temperature rapid cooling conditions are obtained. These recorded parameters are then input into the MK model to predict the forming limit of the specimen under high-temperature rapid cooling. Following the above instructions, a non-isothermal rapid cooling tensile test is finally achieved before Ms (temperature). Figure 2As shown, this sub-experiment aims to explore the synergistic effect of liquid nitrogen jet and non-isothermal deformation: it investigates the interaction between the cooling rate and strain rate when high-temperature sheet metal is rapidly cooled to Ms before the hot stamping quenching and holding pressure process. By obtaining the high-temperature hot forming limit under rapid liquid nitrogen cooling and high strain rate conditions, it can accurately capture the abrupt change in yield strength and the inflection point of forming limit of the material under rapid cooling conditions, thus achieving accurate prediction of high-temperature non-isothermal forming of sheet metal.

[0057] After completing Sub-Experiment 1, Sub-Experiment 2 is conducted. Sub-Experiment 2 uses the same heat treatment process as Sub-Experiment 1, essentially optimizing it further to simultaneously investigate the effects of the process parameters from Sub-Experiment 1 on the sample microstructure while achieving the objectives of Sub-Experiment 2. The method for Sub-Experiment 2 includes the following steps:

[0058] S321, Set I initial temperatures T1, T2, ..., T I The heat-insulated sample was then air-cooled to the i-th initial temperature T. i i∈[1, I]. The sample is air-cooled to the i-th initial temperature T. i Then, the specimen was subjected to strain rate Applying the dependent variable ε i Perform tensioning. While stretching, simultaneously cool the specimen at an average cooling rate V. ij Cool the sample until the temperature drops to the martensitic transformation temperature M. f The sample was then cooled to room temperature at the maximum cooling rate Vmax.

[0059] Among them, T1, T2, ..., T in sub-experiment 2 I , V ij The parameters are consistent with those in sub-experiment one. ε i The determination is based on the following formula:

[0060] ε i =ε iMax +pΔε

[0061]

[0062] In the formula, ε iMax It is the largest pre-dependent variable, and ε iMax Less than A; Δε is the strain interval in the experiment. DT i This is a preset temperature range. P represents different ε values. i The set number of groups, and p∈[1,P].

[0063] Furthermore, it is worth mentioning that during the entire non-isothermal tensile test, the temperature gradient distribution at the gauge length end of the specimen was obtained, and based on the temperature gradient distribution and V... ij , The cooling position and instantaneous cooling rate are adjusted to ensure that the cooling position follows the deformation zone of the sample. For example, when the bottom of the sample is fixed and the top is stretched, the rapid cooling rate of liquid nitrogen means that if the cooling zone remains unchanged, a significant temperature difference will inevitably occur between the top and bottom of the sample, affecting the position of the strain zone. To avoid this problem, the height of the cooling zone needs to be increased to ensure a uniform temperature gradient distribution on the sample. Specifically, the cooling position and liquid nitrogen jet flow rate are determined according to the following formula:

[0064] Q in =Q rad +Q con +Q gas =h tol ·s·△T

[0065]

[0066] d 32 =-g·q 2 -m·q+n

[0067]

[0068]

[0069] In the formula, Q in Q rad Q con Q gas These represent the total heat absorption, thermal radiation from the steel plate, heat absorbed by liquid nitrogen vaporization, and forced convection heat transfer between liquid nitrogen and the steel plate (unit: W), h. tol The comprehensive heat transfer coefficient (unit: W / (m²)) 2 ·℃), where T is temperature (unit: ℃) and s is the heated area (unit: m²). 2 ), ρ, C, V T , q, d 32 v, ρ l ρ v , λ v μ l μ v C pv ,θ,F P S, P1, and P2 are the densities (kg / m³) of the liquid nitrogen before it came into contact with the sample. 3 ), specific heat capacity (J / (kg·℃)), volume (m³) 3 ), flow rate (m 3 / s), liquid nitrogen droplet diameter (m), flow velocity (m / s), and liquid density (kg / m³) when liquid nitrogen comes into contact with the sample and causes evaporation. 3 ), the gas density (kg / m³) when liquid nitrogen comes into contact with the sample and causes evaporation.3 ), thermal conductivity of the gas film (W / (m·K)), viscosity of the liquid when liquid nitrogen comes into contact with the sample to produce evaporation (Pa·s), viscosity of the gas when liquid nitrogen comes into contact with the sample to produce evaporation (Pa·s), specific heat of the gas at constant pressure (J / (kg·K)), latent heat of vaporization (J / (kg·K)), geometric factors, ratio of density of the actual fluid to that of standard water, valve inlet pressure (MPa), outlet pressure (MPa), and a, b, c, e, f, g, m, n are constant coefficients.

[0070] S322. The strain concentration area of ​​the specimen is cut using liquid-cooled wire cutting to obtain the experimental section. It should be noted that cold cutting should be used as much as possible, avoiding methods such as laser cutting, to prevent changes in the microstructure of the experimental section due to high-temperature cutting.

[0071] The cut-off experimental section was subjected to a room-temperature tensile test to obtain mechanical property indicators, and the changes in the microstructure of the sample in sub-experiment one were obtained based on these indicators. The temperature nodes for sub-experiment two were as follows: Figure 3 As shown, specifically, this study explores the effects of rapid liquid nitrogen cooling, high strain, and strain rate on the microstructure changes of high-temperature sheet metal before hot stamping, quenching, and pressure holding processes by examining mechanical performance indicators and microstructure. Ultimately, it seeks to investigate the influence of liquid nitrogen spraying technology on the sheet metal forming quality.

[0072] S323, Traverse T1, T2, ..., T I Obtain I initial temperatures T1, T2, ..., T I The mechanical properties of the samples were investigated. In summary, based on Sub-Experiment 1, Sub-Experiment 2 explored the mechanical properties and microstructure changes of the samples after non-isothermal forming under liquid nitrogen spray.

[0073] In another embodiment, a testing device for non-isothermal liquid nitrogen cooling deformation of high-strength steel is also proposed, which uses the performance testing method for non-isothermal liquid nitrogen cooling deformation of high-strength steel described in the above embodiments. Figure 4 As shown, the testing device mainly includes: a tensile mechanism, a liquid nitrogen jet mechanism, a sensor module, a cutting mechanism, and a controller.

[0074] The tensile mechanism includes a frame, a slide table 1, a servo motor 8, a clamp 11, and two ball screws 13. The two ball screws 13 are vertically connected in parallel to the frame. The slide table 1 is horizontally mounted on the two ball screws 13 and can rise and fall with the forward and reverse rotation of the ball screws 13. The clamp 11 consists of upper and lower parts. The upper part is fixed to the bottom of the slide table 1, and the lower part is fixed to the frame. The upper and lower parts of the clamp 11 respectively clamp the two ends of the sample for tensile testing. The servo motor 8 is fixed to the bottom of the frame and drives one of the ball screws 13 to rotate via a coupling 9. It also drives the other ball screw 13 to rotate synchronously via a synchronous belt 10 and a synchronous pulley structure, thereby controlling the rising and falling of the clamp 11 on the slide table 1. Based on the preset strain rate during the sample test, the rotational speed of the servo motor 8 is calculated according to the sample gauge length and transmission ratio, thus achieving the set strain rate.

[0075] The liquid nitrogen jet mechanism is used for rapid cooling of the sample. It includes a liquid nitrogen tank 7, a solenoid valve 6, a pipe, a nozzle 14, and a triaxial motion module 12. The liquid nitrogen tank 7 contains liquid nitrogen for rapid cooling of the sample and can be sprayed through the pipe and nozzle 14 to the sample fixed by the tensile mechanism. The solenoid valve 6 is installed at the outlet of the liquid nitrogen tank 7 to control the opening and closing of the liquid nitrogen tank 7. The triaxial motion module 12 is used to fix the nozzle 14, adjust the spray direction and position of the nozzle 14, and adjust the displacement of the nozzle 14 in the non-isothermal tensile test to follow the sample cooling deformation area. This ensures that the vertical and horizontal displacement and forward and backward displacement of the nozzle 14 in the non-isothermal tensile test maintain a constant cooling area and cooling effect in the sample cooling deformation area, i.e., maintains a constant temperature gradient of the sample. The triaxial motion module 12 can be a commercially available motor-driven triaxial motion platform, which can be used for follow-up control to precisely adjust the position of the nozzle 1 relative to the sample. Alternatively, a manual adjustment structure can be used to save costs and facilitate operation. In this embodiment, a manually adjustable triaxial motion module 12 is used as an example. Figure 5 As shown, it includes a dovetail slide rail 121, a transverse slide rail 122, and a base 123. The origin of the coordinate system is a point on the sample. The transverse slide rail 122 is movably mounted on the frame and its position is adjustable along the Y direction. The dovetail slide rail 121 is vertically connected to the transverse slide rail 122 and its position is adjustable along the X direction. The base 123 is used to fix the nozzle 14 and is connected to the dovetail slide rail 121 via the dovetail portion. The base 123 is adjusted along the Z direction via a screw connection. When the height of the base 123 needs to be adjusted, the screw is rotated to raise or lower the base 123. For example, when the bottom of the sample is fixed and the top is stretched, the height of the nozzle 14 needs to be raised accordingly to ensure a uniform overall cooling rate. Different average cooling rates of the sample can be adjusted based on the position of the nozzle 14 and the flow rate of liquid nitrogen.

[0076] The monitoring module includes a thermal imager 2, a green light 3, an industrial camera 5, a tension sensor 15, and a flow valve 4. The thermal imager 2 monitors the temperature of the sample. The tension sensor 15, mounted on the fixture 11, monitors the tension applied to the sample. The green light 3 and the industrial camera 5 work together to irradiate and photograph the sample, forming a DIC device. This device effectively identifies high-temperature white speckles on the sample and measures the non-isothermal deformation process under liquid nitrogen injection conditions in real time, which is used for subsequent data analysis to calculate strain. The flow valve 4 is installed on the pipeline to monitor the liquid nitrogen injection flow rate. The cutting mechanism can use an existing wire cutting machine tool, cooled by liquid cooling to avoid excessive temperature rise during the cutting process.

[0077] The controller, based on information collected by the monitoring module, uses a fuzzy adaptive PID algorithm and experimental prediction to control the opening of solenoid valve 6 and the spatial position of nozzle 14, thereby regulating the liquid nitrogen flow rate, injection pressure, and cooling rate during non-isothermal forming. Specifically, the key test parameters during non-isothermal tensile testing are: 1. strain rate; 2. cooling rate. For strain rate, a constant strain rate is crucial for ensuring the accuracy and reliability of material mechanical property test data. In non-isothermal tensile testing, the strain rate is typically calculated based on the rate of displacement change. The cooling rate is mainly controlled by the real-time temperature collected by the monitoring module to adjust the flow coefficient of solenoid valve 6, achieving the specified cooling rate during the non-isothermal process.

[0078] This testing apparatus for non-isothermal liquid nitrogen cooling deformation of high-strength steel can focus on investigating the influence mechanism of non-isothermal rapid cooling deformation performance of high-strength steel from the initial temperature to the martensite initiation temperature (420℃) under different average cooling rates and strain rates. Taking 22MnB5 steel as an example, its continuous cooling transformation (CCT) curve shows that a soft phase transformation occurs at around 700℃, i.e., ferrite or pearlite is generated. To investigate the influence of the soft phase formation ratio on the forming performance during the hot stamping forming transfer time, a temperature range from the initial temperature to 420℃ was set to achieve the study of the predictive performance of non-isothermal rapid cooling deformation under different soft phase ratios.

[0079] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0080] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method for testing the performance of high-strength steel under non-isothermal liquid nitrogen cooling deformation, characterized in that, It employs a liquid nitrogen jet to circulate the deformation region of the sample, rapidly cools the sample, and conducts a non-isothermal tensile test; the non-isothermal tensile test includes: Investigating the temperature at which the sample begins to undergo martensite transformation Ms The first sub-experiment of the previous performance test, which was used to obtain the forming limit of the sample under rapid cooling at high temperature, included the following steps: S311, The sample is air-cooled to the [missing information]. i Initial temperature T i Then, the specimen was subjected to strain rate Perform tensioning; simultaneously cool the specimen at an average rate. V ij Cooling is performed, and the sample temperature is reduced to... Ms Previously, the sample was broken; in, and V ij The relationship between them is satisfied: ； In the formula, t i Cooling time, A The ultimate tensile strain is defined as the austenitic strain under both thermal and isothermal conditions. T f The temperature at which the sample breaks; S312, Traversal T 1. T 2、…、 T I The parameters of the yield criterion during the stretching process are recorded; the recorded parameters are then input into the MK model to predict the forming limit of the specimen under high temperature and rapid cooling. Investigating the sample in Ms Sub-experiment two on performance then includes the following steps: S321, Settings I Initial temperature T 1. T 2、…、 T I ; The heat-insulated sample was air-cooled to the [temperature value missing]. i Initial temperature T i ; i ∈[1, I ]; The sample was air-cooled to the first i Initial temperature T i Then, the specimen was subjected to strain rate Applying the dependent variable Perform tensioning; while stretching, first cool the sample according to the average cooling rate. V ij Cool the sample until the temperature drops to the temperature of complete martensite transformation. M f Then the sample was cooled at the maximum rate. Vmax Cool to room temperature; S322. Cut the strain concentration portion of the specimen to obtain the experimental section; The experimental section was subjected to room temperature tensile tests to obtain mechanical property indicators, and the changes in the microstructure of the sample in sub-experiment one were obtained based on the mechanical property indicators. S323, Traversal T 1. T 2、…、 T I Obtain I initial temperatures T 1. T 2、…、 T I Mechanical properties of the specimens under the specified conditions; In non-isothermal tensile tests, the temperature gradient distribution at the gauge length of the specimen is obtained, and based on the temperature gradient distribution, V ij , Adjust the cooling position and liquid nitrogen jet flow rate to ensure that the cooling position follows the cooling deformation area of ​​the sample; The cooling location and liquid nitrogen jet flow rate are determined according to a formula; the formula is as follows: ； ； ； ； ； ； ； In the formula, Q in Q rad Q con Q gas These are the total heat absorption, the thermal radiation from the steel plate, the heat absorbed by the vaporization of liquid nitrogen, and the forced convection heat transfer between liquid nitrogen and the steel plate, respectively, h. tol The overall heat transfer coefficient is given, where T is the temperature. s For the heated area, ρ, C, V T , q, d 32 v, ρ l ρ v , λ v 、 μ l μ v 、C pv ϴ, F P S, P 1 、P 2 These include, respectively, density, specific heat capacity, volume, flow rate, liquid nitrogen droplet diameter, flow velocity, liquid density, gas density, thermal conductivity of the gas film, liquid viscosity, gas viscosity, isobaric specific heat of gas, latent heat of vaporization, geometric factors, ratio of actual fluid density to standard water density, valve inlet pressure, and outlet pressure. a, b, c, e, f, g, m, n constant coefficients d Indicates the diameter of the pipe nozzle, K v This indicates the valve flow coefficient.

2. The performance testing method for non-isothermal liquid nitrogen cooling deformation of high-strength steel according to claim 1, characterized in that, In S311, if T f ≥ Ms If so, then record the data for that experiment; Conversely, if the data is not recorded, the data for that experiment will not be recorded.

3. The performance testing method for non-isothermal liquid nitrogen cooling deformation of high-strength steel according to claim 1, characterized in that, In S321, I Initial temperature T 1. T 2、…、 T I The method for determining it is as follows: ； In the formula, The interval is the initial temperature, and is an integer. Af The temperature at which the austenite in the sample undergoes a complete transformation is denoted as .

4. The performance testing method for non-isothermal liquid nitrogen cooling deformation of high-strength steel according to claim 1, characterized in that, In S321, V ij The method for determining it is as follows: ； ； ； In the formula, V ij min is the time before the sample breaks. Ms Minimum cooling rate at that time; K ij It is a constant; For cooling intervals; DT i The preset temperature range; M For different V ij The number of groups is set.

5. The performance testing method for non-isothermal liquid nitrogen cooling deformation of high-strength steel according to claim 1, characterized in that, In S321, the method for determining is as follows: In the formula, It is the largest pre-dependent variable and is less than A ; The strain interval of the experiment; DT i The preset temperature range; P For different The set number of groups, and p ∈[1, P ].

6. The performance testing method for non-isothermal liquid nitrogen cooling deformation of high-strength steel according to claim 1, characterized in that, In S322, the sample is cut using wire cutting.

7. A testing device for non-isothermal liquid nitrogen cooling deformation of high-strength steel, comprising: A tensioning mechanism for stretching a specimen, a monitoring module for monitoring the temperature and strain of a specimen, and a cutting mechanism for cutting the strain concentration portion of a specimen. Its characteristic is that it uses the performance testing method for non-isothermal liquid nitrogen cooling deformation of high-strength steel as described in any one of claims 1 to 6; the testing device for non-isothermal liquid nitrogen cooling deformation of high-strength steel further includes a liquid nitrogen jet mechanism, which is used to rapidly cool the sample. The liquid nitrogen jet mechanism includes: a liquid nitrogen tank, a solenoid valve, a nozzle, and a triaxial movement module; the outlet of the liquid nitrogen tank is connected to the nozzle; the nozzle is connected to the triaxial movement module; the solenoid valve is located at the outlet of the liquid nitrogen tank and is used to control the opening and closing of the liquid nitrogen tank; the triaxial movement module is used to controllably adjust the position between the nozzle and the sample, and to adjust the nozzle displacement and the sample cooling deformation area in non-isothermal tensile tests, so that the cooling deformation area and cooling effect of the sample can be kept constant by adjusting the up-down and back-forward displacement of the nozzle in non-isothermal tensile tests.

8. The testing device for non-isothermal liquid nitrogen cooling deformation of high-strength steel according to claim 7, characterized in that, The testing apparatus for non-isothermal liquid nitrogen cooling deformation of high-strength steel also includes: The controller controls the tensioning mechanism, the liquid nitrogen jetting mechanism, and the monitoring module in accordance with the performance testing method for non-isothermal liquid nitrogen cooling deformation of high-strength steel as described in any one of claims 1 to 6, so as to achieve automation.

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