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

Abstract

The invention relates to a performance testing method and a testing device for non-isothermal liquid nitrogen cooling deformation of high-strength steel. According to the performance testing method for the non-isothermal liquid nitrogen cooling deformation of the high-strength steel, liquid nitrogen jet flow is adopted to carry out rapid cooling and follow-up cooling on a sample, and a non-isothermal tensile experiment is carried out. The performance test method comprises the following steps: S1, early-stage preparation; s2, high-temperature heating; and S3, carrying out non-isothermal stretching. According to the method, the high-temperature thermoforming limit can be obtained under the conditions of liquid nitrogen rapid cooling and high strain rate, the yield strength mutation and the forming limit inflection point of the material under the rapid cooling condition can be accurately captured, high-temperature non-isothermal forming of the plate can be accurately predicted, meanwhile, two sub-experiments verify each other, and the accuracy is high. The influence of liquid nitrogen rapid cooling, high strain and strain rate on tissue change before the hot stamping quenching pressure maintaining process of a high-temperature plate is explored from a microstructure, and finally the influence rule of a liquid nitrogen injection process on the forming quality of the plate is explored.

Description

Technical Field

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

[0002] High-strength steel, due to its high strength, high toughness, and fatigue resistance, is widely used in key areas such as automobiles and aerospace. The automotive industry is currently striving to produce high-strength, lightweight components, namely high-temperature thin plate processing. To this end, many scholars are keen to study the thermodynamic parameters and material properties of high-strength steel. However, in the existing technology, such research often uses constant-temperature tensile tests: the steel is cooled to different temperatures according to the experimental design requirements, and then the tensile test is carried out and the data is recorded. In actual production, when forming is carried out using equipment such as roller-hearth heating furnaces and hydraulic presses, static air cooling or water cooling technology is adopted for the steel. At this time, the coordination between the cooling rate and the high strain rate must be considered. Otherwise, the material properties and thermodynamic parameters under non-isothermal deformation at high temperatures will be inaccurately evaluated. For example, high-temperature hot forming limit diagrams are widely used to guide the hot stamping of metal sheets. Usually, high-temperature hot forming limit diagrams obtain the forming limit surface at a constant temperature. This is different from the actual high-temperature hot stamping process, where the sheet temperature continuously changes, making it difficult to accurately guide the hot stamping of metal sheets. Summary of the Invention

[0003] Based on this, it is necessary to provide a performance testing method and testing device for non-isothermal liquid nitrogen cooling deformation of high-strength steel to address the problem that the existing technology does not consider the synergy between cooling rate and high strain rate, resulting in inaccurate evaluation of material properties and thermodynamic parameters during non-isothermal deformation at high temperature.

[0004] In its first aspect, the present invention proposes a method for testing the deformation properties of high-strength steel using non-isothermal liquid nitrogen cooling. This method uses a liquid nitrogen jet to rapidly cool a specimen and then conducts a non-isothermal tensile test. The non-isothermal tensile test includes a first sub-experiment investigating the specimen's performance before the martensitic transformation temperature, Ms, to determine its forming limit under high-temperature rapid cooling, and a second sub-experiment investigating the specimen's performance after Ms.

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

[0006] S311, set I initial temperatures T1, T2, ..., T I ; Air cool the sample after heat preservation to the i-th initial temperature T i ; i∈[1, I]. The sample is air-cooled to the i-th initial temperature T i Afterwards, the sample was strained at a rate Stretching; while stretching, the sample is cooled at an average cooling rate V ij The sample is cooled and the sample is broken before its temperature drops to Ms.

[0007] in, and V ij Satisfaction relationship between:

[0008]

[0009] T f =T i -t i ·V ij

[0010] Where, t i is the cooling time, A is the tensile limit strain of austenite in the isothermal state and the holding state; T f The temperature at which the sample breaks.

[0011] S312, traverse T1, T2, ..., T I , and record the parameters of the yield criterion during the stretching process; the recorded parameters are input into the MK model to predict the forming limit of the sample 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 Afterwards, the sample was strained at a rate Applied strain ε i During stretching, the sample is first cooled at an average cooling rate V ij Cooling is carried out until the sample temperature drops to the martensite complete transformation temperature M f ; Then the sample is cooled to room temperature at the maximum cooling rate Vmax.

[0014] S322. Cut the strain concentration part of the sample to obtain the experimental part; perform a room temperature tensile test on the experimental part to obtain mechanical performance indicators, and obtain the changes in the sample structure in sub-experiment 1 based on the mechanical performance indicators.

[0015] S323, traverse T1, T2, ..., T I , obtain I initial temperatures T1, T2, ..., T I Mechanical properties of the specimens under test.

[0016] In a second aspect, the present invention further provides a testing device for the non-isothermal liquid nitrogen cooling deformation of high-strength steel, which utilizes the performance testing method for the non-isothermal liquid nitrogen cooling deformation of high-strength steel described in the first aspect. The testing device comprises a stretching mechanism for stretching a specimen, a monitoring module for monitoring the temperature and strain of the specimen, a cutting mechanism for cutting strain-concentrated portions of the specimen, and a liquid nitrogen jet mechanism for rapidly cooling the specimen.

[0017] The liquid nitrogen jet mechanism includes: a liquid nitrogen tank, a solenoid valve, a nozzle and a three-axis movable module; the outlet of the liquid nitrogen tank is connected to the nozzle; the nozzle is connected to the three-axis movable module; the solenoid valve is arranged at the outlet of the liquid nitrogen tank to control the opening and closing of the liquid nitrogen tank; the three-axis movable module is used to control the position between the nozzle and the sample and to adjust the nozzle displacement and the sample cooling deformation area to follow the non-isothermal tensile test, so that the up and down displacement and the front and back displacement of the nozzle can be adjusted in the non-isothermal tensile test to achieve a constant cooling area of the sample cooling deformation area and a constant cooling effect.

[0018] The beneficial effects of the present invention are:

[0019] The present invention can obtain the high-temperature hot forming limit under the conditions of rapid cooling of liquid nitrogen and high strain rate, and can accurately capture the yield strength mutation and forming limit inflection point of the material under rapid cooling conditions, and realize accurate prediction of high-temperature non-isothermal forming of the plate. At the same time, the two sub-experiments verify each other, and explore the influence of rapid cooling of liquid nitrogen and high strain and strain rate on the organizational changes of the high-temperature sheet material before the hot stamping quenching and holding process from the microstructure, and finally explore the influence of liquid nitrogen spraying process on the sheet metal forming quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0021] Figure 1 This is a flow chart of the performance testing method for high-strength steel non-isothermal liquid nitrogen cooling deformation;

[0022] Figure 2 Schematic diagram of transient forming limit stretching under high temperature liquid nitrogen in sub-experiment 1;

[0023] Figure 3 Schematic diagram of the pre-strain effect under high-temperature liquid nitrogen in sub-experiment 2;

[0024] Figure 4Schematic diagram of the structure of the testing device for non-isothermal liquid nitrogen cooling deformation of high-strength steel;

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

[0026] In the figure: slide 1, thermal imager 2, green light 3, flow valve 4, industrial camera 5, solenoid valve 6, liquid nitrogen tank 7, servo motor 8, coupling 9, synchronous belt 10, fixture 11, three-axis moving module 12, dovetail slide 121, transverse slide 122, base 123, ball screw 13, nozzle 14, tension sensor 15. DETAILED DESCRIPTION

[0027] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0028] It should be noted that when a component is referred to as being "mounted on" another component, it may be directly on the other component or there may be a central component. When a component is considered to be "set on" another component, it may be directly set on the other component or there may be a central component. When a component is considered to be "fixed to" another component, it may be directly fixed to the other component or there may be a central component.

[0029] It should be understood that the terms "length", "width", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and 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, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on this application.

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

[0031] The present invention achieves rapid cooling of high-strength steel by precisely controlling the pressure, flow, time, and position of the liquid nitrogen jet, utilizing the low-temperature characteristics of liquid nitrogen. Simultaneously, the strain, strain rate, and forming temperature range of the test device are adjusted to control the rapid cooling non-isothermal tensile deformation process of high-temperature high-strength steel. Through this multi-parameter coupling processing method, deformation control is combined with the high-temperature rapid cooling process to optimize the non-isothermal forming performance of high-strength steel under liquid nitrogen spraying. By deeply analyzing the tensile data of high-strength steel at different strains and strain rates under different cooling paths and within a specified temperature range, it is possible to fully explore the effects of different non-isothermal processes on the formed surface of high-strength steel.

[0032] For details, please refer to Figure 1 This embodiment provides a method for testing the deformation performance of high-strength steel using non-isothermal liquid nitrogen cooling. This method uses a liquid nitrogen jet to rapidly cool the specimen and then conducts a non-isothermal tensile test. This performance testing method includes the following steps: S1, preliminary preparation; S2, high-temperature heating; S3, non-isothermal stretching. Each step is described below:

[0033] S1. Preliminary preparation.

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

[0035] Among them, the strain measurement preparation includes speckle creation and camera calibration. First, a suitable speckle is created on the surface of the sample, and then digital image correlation (DIC) is used to calculate the displacement and strain by analyzing the grayscale information of the sample surface before and after deformation. The digital image correlation used is a method based on non-contact optical measurement, which is widely used to obtain deformation displacement and strain information of objects, especially suitable for measurement in high temperature environments. During the strain measurement process, white high-temperature resistant paint is sprayed at room temperature and combined with green light illumination and filters of corresponding wavelengths to effectively suppress blackbody radiation interference to improve the grayscale of the material surface, ensuring high-precision recognition and measurement of DIC in high-temperature environments below 1000°C.

[0036] Temperature measurements are prepared using either a contact thermocouple or a non-contact thermal imager. If using a thermocouple, first sand the specimen surface with sandpaper to ensure good contact between the thermocouple and the specimen. Then, securely weld the thermocouple to the specimen using a butt welder. Finally, cure the specimen 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 fully austenitic cooling, the experiment required ensuring that the initial deformed grains were fully austenitic. In this step, the specimens were uniformly heated in a resistance heating furnace at a rate of 15°C / s to a constant temperature of 920°C, where they were held for 5 minutes.

[0039] S3. Non-isothermal stretching.

[0040] This step aims to explore the mechanical properties of the austenite during the transformation to martensite under non-isothermal liquid nitrogen cooling conditions, focusing on the changes in non-isothermal tensile properties. By setting a specific temperature range: initial temperature T i , martensite start transformation temperature Ms (about 420℃) and martensite complete transformation temperature M f (about 200℃), study the liquid nitrogen flow rate (cooling effect) V ij , strain rate and the dependent variable ε i Impact on Material Mechanical Properties. In this embodiment, the non-isothermal tensile test includes: Sub-experiment 1, which investigates the performance of the specimen before the martensitic transformation temperature, Ms, to determine the forming limit of the specimen under high-temperature rapid cooling; and Sub-experiment 2, which investigates the performance of the specimen after Ms. Sub-experiment 2 also investigates how the process parameters in Sub-experiment 1 affect the specimen structure.

[0041] Among them, sub-experiment 1 includes the following steps:

[0042] S311, set I initial temperatures T1, T2, ..., T I The sample after heat preservation is 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 Afterwards, the sample was strained at a rate Stretching; while stretching, the sample is cooled at an average cooling rate V ij The sample is cooled and the sample is broken before its temperature drops to Ms.

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

[0044] T i =Ms+iΔT

[0045]

[0046] Where ΔT is the interval of initial temperature and is rounded to an integer. Af is the austenite complete transformation temperature of the sample.

[0047] according to The different states can be divided into different states: quasi-static strain rate, the range of which is 10 -5 ~10 -1 S -1 , dynamic strain rate, including medium strain rate: 1 to 10 2 S -1, High strain rate: 10 2 ~10 4 S -1 , Ultra-high strain rate: >10 4 S -1 . At the same time, it is necessary to control the liquid nitrogen flow to achieve V ij V ij The determination method is based on the following formula:

[0048] V ij =V ij min+K ij ΔV

[0049]

[0050]

[0051] Where V ij min is the minimum cooling rate before the sample breaks. K ij is a constant. ΔV is the cooling interval. DT i is the preset temperature range. M is different V ij Set the number of groups.

[0052] Confirm V ij After that, and V ij Satisfaction relationship between:

[0053]

[0054] T f =T i -t i ·V ij

[0055] Where, t i is the cooling time. A is the tensile limit strain of austenite in the isothermal state. T f is the temperature when the sample breaks. It should be noted that if T f If the value is ≥ Ms, the data of this experiment will be recorded. Otherwise, the data set is unreliable and will not be recorded.

[0056] S312, traverse T1, T2, ..., T I , obtain the parameters of the yield criterion under high temperature rapid cooling conditions. Input the recorded parameters into the MK model to predict the forming limit of the specimen under high temperature rapid cooling. According to the above instructions, the non-isothermal rapid cooling tensile test is finally achieved before Ms. Figure 2As shown, this sub-experiment seeks to investigate the synergy between liquid nitrogen jet and non-isothermal deformation: it investigates the interaction between cooling rate and strain rate when a high-temperature sheet material is rapidly cooled to Ms prior to the quenching and holding process in hot stamping. By obtaining the high-temperature hot forming limit under rapid liquid nitrogen cooling and high strain rates, it accurately captures the material's yield strength mutation and forming limit inflection point under rapid cooling conditions, enabling precise prediction of high-temperature non-isothermal forming of sheet metal.

[0057] After completing Sub-Experiment 1, Sub-Experiment 2 was conducted. Sub-Experiment 2 used the same heat treatment process as Sub-Experiment 1, but was optimized based on Sub-Experiment 1. This allowed us to simultaneously explore the effects of the process parameters in Sub-Experiment 1 on the microstructure of the sample while achieving the objectives of Sub-Experiment 2. The method for Sub-Experiment 2 included the following steps:

[0058] S321, set I initial temperatures T1, T2, ..., T I The sample after heat preservation is 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 Afterwards, the sample was strained at a rate Applied strain ε i During stretching, the sample is first cooled at an average cooling rate V ij Cooling is carried out until the sample temperature drops to the martensite complete transformation temperature M f The sample is then cooled to room temperature at the maximum cooling rate Vmax.

[0059] Among them, T1, T2, ..., T I 、 V ij The parameters are consistent with those in sub-experiment 1. i The determination is based on the following formula:

[0060] ε i =ε iMax +pΔε

[0061]

[0062] Where, ε iMax is the maximum pre-strain variable, and ε iMax Less than A; Δε is the strain interval of the experiment. DT i is the preset temperature range. P is different ε i The number of set groups, and p∈[1,P].

[0063] In addition, it is worth mentioning that during the entire non-isothermal tensile test, the temperature gradient distribution at the gauge end of the specimen is obtained, and according to the temperature gradient distribution, V ij 、 Adjust the cooling position and instantaneous cooling rate so that the cooling position and the cooling deformation area of the sample follow each other. For example, when the bottom end of the sample is fixed and the upper end is stretched and lengthened, due to the faster cooling rate of liquid nitrogen, if the cooling area remains unchanged at this time, it will inevitably cause a significant temperature difference between the upper and lower ends of the sample, affecting the position of the sample strain area. In order to avoid the above problem, the height of the cooling area needs to be raised to make the temperature gradient of the sample uniform. Specifically, the cooling position and the liquid nitrogen jet flow rate are determined according to the relationship formula, and the relationship formula is:

[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] Where Q in , Q rad , Q con , Q gas They are total heat absorption, heat radiation of steel plate, heat absorbed by liquid nitrogen vaporization, and forced convection heat transfer between liquid nitrogen and steel plate (unit: W), h tol is the comprehensive heat transfer coefficient (unit: W / (m 2 ·℃), T is temperature (unit: ℃), s is the heated area (unit: m 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 of liquid nitrogen before contact with the sample (kg / m 3 ), specific heat capacity (J / (kg·℃)), volume (m 3 ), flow rate (m 3 / s), liquid nitrogen droplet diameter (m), flow rate (m / s), liquid density when liquid nitrogen contacts the sample and evaporates (kg / m 3 ), gas density when liquid nitrogen contacts the sample and evaporates (kg / m3 ), thermal conductivity of gas film (W / (m·K)), viscosity of liquid when liquid nitrogen contacts the sample and evaporates (Pa·s), viscosity of gas when liquid nitrogen contacts the sample and evaporates (Pa·s), specific heat of gas at constant pressure (J / (kg·K)), latent heat of vaporization (J / (kg·K)), geometric factors, density ratio of actual fluid to that under standard water, valve inlet pressure (MPa), outlet pressure (MPa), a, b, c, e, f, g, m, n are constant coefficients.

[0070] S322. Use liquid-cooled wire cutting to cut the strain concentration portion of the specimen to obtain the experimental portion. It should be noted that cold cutting should be used as much as possible to avoid laser cutting, as this can cause changes in the experimental portion's structure due to high-temperature cutting.

[0071] The cut experimental part is subjected to room temperature tensile test to obtain mechanical performance index, and the change of sample structure in sub-experiment 1 is obtained based on the mechanical performance index. Figure 3 As shown in the figure, specifically, by exploring the effects of rapid liquid nitrogen cooling of high-temperature sheet metal before the hot stamping quenching and holding process as well as high strain and strain rate on the organizational changes from the perspectives of mechanical properties and microstructure, we finally explore the influence of liquid nitrogen jetting process on the forming quality of sheet metal.

[0072] S323, traverse T1, T2, ..., T I , obtain I initial temperatures T1, T2, ..., T I In general, 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 jet.

[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 in the above embodiment. Figure 4 As shown, the testing device mainly includes: a stretching mechanism, a liquid nitrogen jet mechanism, a sensor module, a cutting mechanism and a controller.

[0074] The stretching mechanism includes a frame, a slide 1, a servo motor 8, a clamp 11, and two ball screws 13. The two ball screws 13 are connected to the frame in parallel in the vertical direction. The slide 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 is divided into two parts, the upper part is fixed to the bottom of the slide 1. The lower part is fixed to the frame. The upper and lower parts of the clamp 11 clamp the two ends of the sample for stretching. The servo motor 8 is fixed to the bottom of the frame, and it drives one of the ball screws 13 to rotate through the coupling 9, and drives the other ball screw 13 to rotate synchronously through the synchronous belt 10 and the synchronous wheel structure, thereby controlling the lifting and lowering of the clamp 11 on the slide 1. Based on the strain rate preset during the sample experiment, the speed of the servo motor 8 is calculated according to the sample gauge length and transmission ratio, and the set strain rate can be achieved.

[0075] The liquid nitrogen jet mechanism is used to quickly cool the sample, and includes a liquid nitrogen tank 7, a solenoid valve 6, a pipe, a nozzle 14, and a three-axis moving module 12. The liquid nitrogen tank 7 is filled with liquid nitrogen for quickly cooling the sample, and can be sprayed to the sample fixed by the stretching mechanism through the pipe and the nozzle 14. 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 three-axis moving 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 to follow the cooling and deformation area of the sample in the non-isothermal stretching experiment, so that the nozzle 14 is adjusted in the non-isothermal stretching experiment. The up and down displacement and the front and back displacement of the nozzle 14 are adjusted to achieve a constant cooling area of the cooling deformation area of the sample and a constant cooling effect, that is, to keep the temperature gradient of the sample constant. The three-axis moving module 12 can adopt a commercially available three-axis motion platform driven by a motor, and can perform follow-up control to accurately adjust the position of the nozzle 1 relative to the sample, or a manual adjustment structure can be used to save costs and facilitate operation. In this embodiment, taking the three-axis moving module 12 with a manual adjustment structure as an example, as Figure 5 As shown, it includes a dovetail slide 121, a transverse slide 122, and a base 123. A certain point on the sample is taken as the coordinate origin. Among them, the transverse slide 122 is movably set on the frame and can adjust its position along the Y direction. The dovetail slide 121 is vertically connected to the transverse slide 122 and can adjust its position along the X direction. The base 123 is used to fix the nozzle 14 and is connected to the dovetail slide 121 through the dovetail. The base 123 is adjusted in the Z direction by a screw connection. When the height of the base 123 needs to be adjusted, the base 123 is raised and lowered by rotating the screw. For example, when the bottom end of the sample is fixed and the upper end is stretched and lengthened, in order to ensure that its overall cooling rate is uniform, the height of the nozzle 14 needs to be raised accordingly. 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 is used to monitor the temperature of the sample. The tension sensor 15 is installed on the fixture 11 to monitor the tension on the sample. The green light 3 and the industrial camera 5 cooperate to illuminate and photograph the sample, forming a DIC device, which can effectively identify high-temperature resistant white speckles on the sample, and measure the non-isothermal deformation process under liquid nitrogen injection conditions in real time, which is used for subsequent data analysis to calculate the strain. The flow valve 4 is installed on the pipeline to monitor the injection flow of liquid nitrogen. The cutting mechanism can use an existing wire cutting machine tool and adopt liquid cooling to avoid excessive temperature rise during the cutting process.

[0077] Based on the information collected by the monitoring module, the controller can control the opening of the solenoid valve 6 and the spatial position of the nozzle 14 through the fuzzy adaptive PID algorithm and experimental prediction to achieve the regulation of liquid nitrogen flow, injection pressure and cooling rate in non-isothermal forming. Specifically, the important factors of test parameters during the non-isothermal tensile test are: 1. Strain rate; 2. Cooling rate. For strain rate, a constant strain rate is a key factor to ensure the accuracy and reliability of material mechanical property test data. In non-isothermal tensile tests, the strain rate is usually calculated based on the rate of change of displacement. The cooling rate mainly relies on the temperature collected in real time by the monitoring module to control the flow coefficient of the solenoid valve 6 to achieve the specified cooling rate in the non-isothermal process.

[0078] The testing device for non-isothermal liquid nitrogen cooling deformation of high-strength steel can focus on the influence mechanism of non-isothermal rapid cooling deformation performance of high-strength steel from the initial temperature to the martensite start transformation temperature (420℃) at different average cooling rates and strain rates. Taking the material 22MnB5 steel as an example, it can be seen from its continuous cooling transformation (CCT) curve that there is a soft phase transformation at around 700℃, that is, ferrite or pearlite is generated. In order to explore the influence of the soft phase generation ratio during the hot stamping transfer time on the forming performance, a temperature range from the initial temperature to 420℃ was set to realize the non-isothermal rapid cooling deformation prediction performance study under different soft phase ratios.

[0079] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned 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 above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A method for testing the deformation performance of high-strength steel by non-isothermal liquid nitrogen cooling, characterized in that: It uses a liquid nitrogen jet to follow the deformation area of the sample, quickly cool the sample, and perform a non-isothermal tensile test; the non-isothermal tensile test includes: Sub-experiment 1: exploring the performance of the specimen before the martensite transformation temperature Ms, which is used to obtain the forming limit of the specimen under high temperature and rapid cooling; and Sub-experiment 2, which explores the performance of the sample after Ms, includes the following steps: S321, set I initial temperatures T1, T2, ..., T I ; The sample after heat preservation is 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 Afterwards, the sample was strained at a rate Applied strain ε i During stretching, the sample is first cooled at an average cooling rate V ij Cooling is carried out until the sample temperature drops to the martensite complete transformation temperature M f ;Then the sample is cooled to room temperature at the maximum cooling rate Vmax; S322, cutting the strain concentration part of the specimen to obtain the experimental part; The experimental part is subjected to room temperature tensile test to obtain mechanical properties, and the changes in the sample structure in sub-experiment 1 are obtained based on the mechanical properties; S323, traverse T1, T2, ..., T I , obtain I initial temperatures T1, T2, ..., T I Mechanical properties of the specimens under test.

2. The performance testing method for high-strength steel non-isothermal liquid nitrogen cooling deformation according to claim 1, characterized in that: The method of sub-experiment 1 includes the following steps: S311, the sample is air-cooled to the i-th initial temperature T i Afterwards, the sample was strained at a rate Stretching; while stretching, the sample is cooled at an average cooling rate V ij Cooling is carried out, and the sample is broken before the sample temperature drops to Ms; in, and V ij Satisfaction relationship between: T f =T i -t i ·V ij Where, t i is the cooling time, A is the tensile limit strain of austenite in the isothermal state and the holding state; T f The temperature at which the sample breaks; S312, traverse T1, T2, ..., T I , and record the parameters of the yield criterion during the stretching process; the recorded parameters are input into the MK model to predict the forming limit of the sample under high temperature and rapid cooling.

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

4. The performance testing method for high-strength steel non-isothermal liquid nitrogen cooling deformation according to claim 1, characterized in that: In S321, the initial temperatures T1, T2, ..., T I The method of determining is: T i =Ms+iΔT Where ΔT is the interval of initial temperature and is an integer; Af is the austenite complete transformation temperature of the sample.

5. The performance testing method for high-strength steel non-isothermal liquid nitrogen cooling deformation according to claim 1, characterized in that: In S321, V ij The method of determining is: V ij =V ij min+K ij ΔV Where V ij min is the minimum cooling rate before the sample breaks and Ms is reached; K ij is a constant; ΔV is the cooling interval; DT i is the preset temperature range; M is different V ij Set the number of groups.

6. The performance testing method for high-strength steel non-isothermal liquid nitrogen cooling deformation according to claim 1, characterized in that: In S321, ε i The method of determining is: e i =e iMax +pNo Where, ε iMax is the maximum pre-strain variable, and ε iMax Less than A; Δε is the strain interval of the experiment; DT i is the preset temperature range; P is different ε i The number of set groups, and p∈[1,P].

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

8. The method for testing the deformation performance of high-strength steel by non-isothermal liquid nitrogen cooling according to claim 1, characterized in that: In the non-isothermal tensile test, the temperature gradient distribution at the gauge end of the specimen is obtained, and according to the temperature gradient distribution, V ij 、 Adjust the cooling position and liquid nitrogen jet flow rate so that the cooling position moves with the cooling deformation area of the specimen; The cooling position and the liquid nitrogen jet flow rate are determined according to the relationship formula; the relationship formula is: Q in =Q rad +Q con +Q gas =h tol ·s·ΔT d 32 =-g·q 2 -m·q+n Where Q in , Q rad , Q con , Q gas They are the total heat absorption, heat radiation of the steel plate, heat absorbed by liquid nitrogen vaporization, and forced convection heat transfer between liquid nitrogen and steel plate, h tol is the comprehensive heat transfer coefficient, T is temperature, s is 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, P1, and P2 are density, specific heat capacity, volume, flow rate, liquid nitrogen droplet diameter, flow rate, liquid density, gas density, thermal conductivity coefficient of gas film, liquid viscosity, gas viscosity, constant pressure specific heat of gas, latent heat of vaporization, geometric factors, ratio of density of actual fluid to that of standard water, valve inlet pressure, and outlet pressure, respectively. a, b, c, e, f, g, m, and n are constant coefficients.

9. A testing device for non-isothermal liquid nitrogen cooling deformation of high-strength steel, comprising: A stretching mechanism for stretching the sample, a monitoring module for monitoring the temperature and strain of the sample, and a cutting mechanism for cutting the strain-concentrated portion of the sample; Characterized in that it uses the performance testing method of high-strength steel non-isothermal liquid nitrogen cooling deformation according to any one of claims 1 to 8; the testing device for high-strength steel non-isothermal liquid nitrogen cooling deformation also includes a liquid nitrogen jet mechanism for rapidly cooling the sample; The liquid nitrogen jet mechanism includes: a liquid nitrogen tank, a solenoid valve, a nozzle and a three-axis movable module; the outlet of the liquid nitrogen tank is connected to the nozzle; the nozzle is connected to the three-axis movable module; the solenoid valve is arranged at the outlet of the liquid nitrogen tank to control the opening and closing of the liquid nitrogen tank; the three-axis movable module is used to control the position between the nozzle and the sample and to adjust the nozzle displacement and the sample cooling deformation area to follow the non-isothermal tensile test, so that the up and down displacement and the front and back displacement of the nozzle can be adjusted in the non-isothermal tensile test to achieve a constant cooling area of the sample cooling deformation area and a constant cooling effect.

10. The high-strength steel non-isothermal liquid nitrogen cooling deformation testing device according to claim 9, characterized in that: The test equipment for non-isothermal liquid nitrogen cooling deformation of high-strength steel also includes: A controller controls the stretching mechanism, the liquid nitrogen jet mechanism, and the monitoring module according to the performance testing method for non-isothermal liquid nitrogen cooling deformation of high-strength steel as described in any one of claims 1 to 8 to achieve automation.

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