CN108279175B - A test device and method for obtaining the differential temperature forming limit of variable-strength steel plate - Google Patents

Abstract

The invention provides a test device for obtaining the differential temperature forming limit of a variable-strength steel plate, comprising a punch consisting of a punch main body, an insert and a ceramic heat insulation layer; Connected together; the insert is fixed at the bottom of the punch and forms a smooth spherical surface with the main body of the punch; the inside of the punch is provided with a deep hole for installing the heating rod and thermocouple; the top of the punch is provided with a threaded hole for connecting the connecting rod ; High-frequency induction heating coils and infrared probes are respectively set around the convex and concave mold blanking rings to form a closed-loop control system; it also includes an upper support plate, a lower support plate, a slider, a guide post, a pillar, a base, a master cylinder, and a blank holder. , light sources, high-frequency cameras and thermal imaging cameras. The invention controls the temperature on both sides of the punch, so that the high-temperature test sample has different quenching rates on both sides of the punch, thereby simulating the differential temperature forming process of the variable-strength steel plate, and obtains the variable-strength through the strain field collected by the high-frequency camera. The differential temperature forming limit of steel plate.

Description

A test device and method for obtaining the differential temperature forming limit of variable-strength steel plate

technical field

The invention relates to the field of hot forming of high-strength steel plates, in particular to a test device and method for obtaining the differential temperature forming limit of variable-strength steel plates.

Background technique

The rapid development of the automobile industry has greatly improved people's travel efficiency and promoted social development, but it has also brought many safety and environmental problems. The realization of lightweight design on the basis of ensuring safety can effectively reduce the fuel consumption and exhaust emissions of automobiles, which is an inevitable trend for the future development of automobiles. Optimizing the structural design of automobiles, using new materials and corresponding advanced manufacturing technologies are the main methods to achieve lightweight design.

Ultra-high-strength steel plate hot forming technology is an advanced manufacturing technology specially used for forming high-strength steel plates. The general process is to heat the steel plate to above 900 ° C to make it fully austenitized, and then rapidly stamping and pressure-holding quenching. , to obtain high-strength components with uniform martensitic structure. The use of ultra-high-strength steel hot forming technology can obtain parts with a strength of more than 1500MPa, so the thickness of the components can be appropriately reduced without reducing the strength of the parts, so as to achieve the purpose of reducing the weight of automobiles.

For some safety components of automobiles, such as B-pillars, door inner panels, etc., the components are required to have sufficient strength to prevent serious damage to the automobile structure, and at the same time, some parts of the components are required to have lower strength and higher energy absorption features, thereby improving the overall crash safety of the vehicle. The traditional hot forming technology can only obtain ultra-high-strength steel components with a single performance, which cannot meet the requirements of different mechanical properties of different parts of the component; by changing the quenching rate of different parts of the component during the forming process, different microstructures and mechanical properties can be obtained. The performance variable-strength structural parts can be used in the body to maximize the overall crash safety of the car.

For the evaluation of the formability of thermoformed sheets, the most commonly used and intuitive method is the Forming Limit Diagram (FLD, or FLC). The so-called forming limit diagram refers to a curve formed by the strain points in the primary and secondary strain coordinate system when the material begins to shrink under different (approximate) linear strain paths during the forming process. The existing related test devices and methods are mainly used to obtain the isothermal forming limit of high-strength steel plates. For example, a Chinese patent (application number 201410076406.8) discloses an experimental device and test method for the hot forming limit of metal plates. Heating the sheet and the punch can prevent the heat transfer between the sheet and the punch during the test, thereby reducing the test error. However, the actual forming of the variable-strength steel plate is a non-isothermal process, and the temperature changes continuously; at the same time, different positions of the variable-strength steel plate have different quenching rates, which is a differential temperature forming process. Therefore, the above devices and methods cannot obtain the difference of the variable-strength steel plate. Warm forming limit. The Chinese patent (application number 201210192708.2) discloses a numerical simulation prediction method for the hot forming transient forming limit of ultra-high strength steel. After parameters, the M-K model for the prediction of the hot forming limit under steady-state conditions is established by using the M-K theory. Based on the determined model, the forming limit under different hot forming transient process conditions is calculated and predicted.

To sum up, at present, there is no relevant report on the test device and method for the differential temperature forming process of variable strength steel plate, and a special test device and method are urgently needed to realize the differential temperature forming process of variable strength steel plate, and test Its formability limit, and its formability are evaluated.

SUMMARY OF THE INVENTION

According to the technical problems existing in the existing forming limit test equipment and methods proposed above, a test device and method for obtaining the differential temperature forming limit of a variable strength steel plate are provided. The test device provided by the invention can simulate the forming process of the variable-strength steel plate and obtain the variable-strength components with specific microstructure and mechanical property distribution; at the same time, the test device can obtain the strain field in the forming process of the steel plate in real time through the integrated optical strain measurement system , to establish an accurate forming limit diagram of variable strength steel plates.

The technical means adopted in the present invention are as follows:

A test device and method for obtaining the differential temperature forming limit of a variable-strength steel plate, comprising:

The punch is mainly composed of two punch bodies with symmetrical structures, an insert and a ceramic heat insulation layer; the ceramic heat insulation layer is installed in the middle of the symmetrical structures of the two punch bodies to isolate the punch Heat transfer on both sides of the head; the punch body and the ceramic thermal insulation layer are tightly connected by bolts; the insert is fixed at the bottom of the punch by bolts, and the punch body forms a smooth transition At the same time, it plays the role of protecting the ceramic thermal insulation layer; the inside of the punch is also provided with deep holes for installing heating rods and thermocouples; the top of the punch is provided with a threaded hole on the top of the punch for connecting the connecting rod , the punch is connected with the master cylinder through the connecting rod;

The hemming ring is nested on the outside of the punch and arranged up and down. Induction heating coil and infrared probe, the infrared probe and the high-frequency induction heating coil form a closed-loop control system, which is used to rapidly heat the blank holder and keep the blank holder at the specified test temperature;

The support structure is mainly composed of a base, a pillar fixed on the base, and a support body for fixing the punch; the support body includes an upper support plate, a guide post, a slider and a lower support plate; the The female die pressing ring is fixed on the lower support plate by bolts, and the male die pressing ring is fixed on the slider by bolts; the sliding block is connected with the pressing cylinder, which drives the male pressing ring along The guide column moves up and down; the upper end of the guide column is connected with the upper support plate by threads, and the lower end of the guide column is fixed on the lower support plate by threads; the master cylinder and the blank holder are bolted It is fixed on the upper support plate, and provides power for the punch and the punch edge pressing ring respectively; the upper end of the strut is connected to the lower support plate through a thread, and the lower end of the strut is connected to the base through a thread ;

The middle of the base is also provided with an infrared thermal imager, which is used for real-time acquisition and recording of the temperature field distribution during the deformation process of the test sample; the two sides of the infrared thermal imager are provided with light sources and high-frequency cameras for real-time Acquire and record the strain field distribution of the test specimen.

Further, an asbestos pad is provided at the connection between the connecting rod and the punch to prevent the heat of the punch from being transferred to the master cylinder through the connecting rod.

Further, cooling water channels are provided inside the upper support plate and the lower support plate to prevent the entire device from being deformed due to excessive temperature.

Further, a circular through hole is provided in the middle of the lower support plate, and the through hole serves as a window for collecting the temperature field and the strain field.

Further, rounded corners are provided at the intersections of one side surface of the male die pressing ring and the female die pressing ring which are in contact with the test sample and the inner wall.

Further, a circular draw bead is provided on the side where the male die holder is in contact with the test sample, and the draw bead matches the groove on the female die holder to prevent the test sample from flowing; Rounded corners are provided at the junction of the drawing bead and the surface of the male die pressing ring; and rounded corners are provided at the junction of the groove and the surface of the female die pressing ring.

The invention discloses a method for obtaining the differential temperature forming limit of a variable-strength steel plate by using the above-mentioned test device, which is characterized by comprising the following steps:

S1. Prepare a sample, spray the surface of the test sample with black and white high temperature resistant speckles, riveted a thermocouple on the edge of the test sample, place the test sample in a heating furnace to heat up to a specified temperature, and keep it for a period of time;

S2. Coat the punch with high temperature resistant grease, the punch edge ring goes down, and the concave die edge ring is pressed to reduce the heat loss during the heating process;

S3. Turn on the high-frequency induction heating coil, and quickly heat the blank holder to the test temperature. The temperature of the blank holder is collected by an infrared probe, and closed-loop control is realized with the high-frequency induction coil; turn on the heating rods on both sides of the punch, or only Turn on the heating rod on one side, and adjust the power of the heating rod to make the two sides of the punch have different temperatures. The temperature of the punch is collected by the thermocouples on both sides and closed-loop control is realized;

S4. Turn on the light source, high-frequency camera and infrared thermal imager;

S5. After the test sample is heated to the specified temperature and kept warm, the punch edge holder moves upward until the test sample can be accommodated to enter the gap between the punch die holder ring and the female die holder ring; The arm quickly transfers the heated test sample out of the heating furnace and places it at the designated position on the die pressing ring;

S6. Quickly close the mold, and the punch pressing ring presses the test sample on the female die pressing ring;

S7. The speed of the punch is controlled in sections. The punch first goes down rapidly until it is about to contact the test sample, and the speed is adjusted to the set test speed; the punch punches the test sample until the test sample shrinks or breaks. At the same time, the temperature field distribution of the test sample is recorded in real time by an infrared thermal imager, and the strain field distribution of the test sample is recorded in real time by a high-frequency camera;

S8. Extract the strain value at the critical rupture moment, and obtain the ultimate strain at the rupture through interpolation;

S9, the punch edge holder and the punch go up, the edge holder ring is opened, and the sample after the test is taken out;

S10. Repeat steps S1-S9 to perform forming limit tests under different strain paths, temperatures, strain rates, etc., to establish differential temperature forming limit curves of the variable-strength steel plate under different test conditions.

The present invention has the following advantages:

1) The test device of the present invention can heat the two sides of the punch to different temperatures, and after the punch contacts the sample, the two sides of the sample can have different quenching rates, thereby simulating the forming process of the variable-strength steel plate;

2) The test device of the present invention heats the temperature of the blank holder to the same temperature as that of the sample to prevent heat exchange with the blank holder after the sample is pressed, which affects the test accuracy;

3) The test device of the present invention uses a high-frequency induction coil to heat and heat the blank holder, with high heating efficiency, uniform temperature distribution, and good heat preservation effect;

4) The test device of the present invention uses a high-frequency camera to acquire and record the strain field distribution during the deformation process of the sample and the strain value at the critical rupture moment in real time, which is convenient, fast and has high accuracy;

5) The forming limit test that can be carried out by the test device of the present invention is not limited to the variable-strength steel plate, but can also carry out ordinary non-isothermal forming limit test. At this time, it is only necessary to set the temperature on both sides of the punch to the same temperature;

6) The test device of the present invention can also be used to carry out the isothermal forming limit test within a certain temperature range, at this time, the punch, the blank holder and the sample only need to be heated to the same temperature.

Based on the above reasons, the present invention can be widely applied in the field of hot forming of high-strength steel sheets.

Description of drawings

In order to illustrate the embodiments of the present invention or the technical solutions in the prior art more clearly, the following briefly introduces the accompanying drawings that need to be used in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description These are some embodiments of the present invention, and for those of ordinary skill in the art, other drawings can also be obtained from these drawings without any creative effort.

Fig. 1 is a longitudinal sectional view of the center of the punch, the punch press edge ring and the female die press edge ring according to the present invention.

Figure 2 is a front view of the punch according to the present invention.

Figure 3 is a top view of the punch according to the present invention.

Figure 4 is a left side view of the punch according to the present invention.

FIG. 5 is a cross-sectional view taken along the line A-A in FIG. 3 .

FIG. 6 is a schematic diagram of the assembly of the punch according to the present invention.

FIG. 7 is a schematic structural diagram of the test device according to the present invention.

Figure 8 is a flow chart of the test method of the present invention.

In the figure: 1. Main body of punch; 2. Ceramic heat insulation layer; 3. Punch pressing ring; 4. Female pressing ring; 5. Insert; 6. Groove; 7. Insert thread hole; 8. Pull Beads; 9. Symmetrical threaded holes; 10. Threaded holes at the top of the punch; 11. Installation holes for heating rods; 12. Installation holes for thermocouples; 13. High-frequency induction heating coils; 14. Asbestos pads; 15. Connecting rods; 16, punch; 17, slider; 18, guide post; 19, upper support plate; 20, master cylinder; 21, blank holder; 22, infrared probe; 23, lower support plate; 24, pillar; 25, base , 26, light source; 27, high frequency camera; 28, infrared thermal imager.

Detailed ways

In order to make the purposes, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments These are some embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

As shown in Figure 7, a test device and method for obtaining the differential temperature forming limit of a variable-strength steel plate, including:

The punch 16 (as shown in Figures 1-5) is mainly composed of two punch bodies 1 with symmetrical structures, an insert 5 and a ceramic heat insulation layer 2; the ceramic heat insulation layer 5 is installed on the two places. The middle of the symmetrical structure of the punch body 1 is used to isolate the heat transfer on both sides of the punch body 16; The structural threaded holes 9 are thus tightly connected together; the insert 5 is fixed on the bottom of the punch 16 through the insert threaded hole 7 by bolts, and forms a transitional smooth spherical surface with the punch body 1, and at the same time plays a role in The function of protecting the ceramic heat insulation layer 2; the punch 16 is also provided with a heating rod installation hole 11 for installing a heating rod and a thermocouple installation hole 12 for installing a thermocouple; the top of the punch 16 is provided with a hole for The threaded hole 10 at the top of the punch is connected to the connecting rod 15, and the punch 16 is connected to the master cylinder 20 through the connecting rod 15; an asbestos pad 14 is provided at the connection between the connecting rod 15 and the punch 16, which is used for The heat of the punch 16 is prevented from being transferred to the master cylinder 20 through the connecting rod 15 .

The edge blanking ring (as shown in FIG. 6 ) is nested on the outside of the punch 16 and arranged up and down, and is divided into a punch blanking ring 3 and a female die blanking ring 4 . A high-frequency induction heating coil 13 and an infrared probe 22 are respectively arranged around the die pressing ring 4. The infrared probe 22 and the high-frequency induction heating coil 13 form a closed-loop control system for rapidly heating the pressing ring and maintaining the pressure. The edge ring is at the specified test temperature;

The support structure is mainly composed of a base 25 , a pillar 24 fixed on the base 25 , and a support body for fixing the punch 16 ; the support body includes an upper support plate 19 , a guide post 18 , and a slider 17 and the lower support plate 23; the female die pressing ring 4 is fixed on the lower supporting plate 23 by 6 bolts, and the male die pressing ring 3 is fixed on the slider 17 by 6 bolts; The block 17 is connected to the edge blanking cylinder 21, and drives the punch edge blanking ring 3 to move up and down along the guide post 18; The upper support plate 19 and the lower support plate 23 are both provided with cooling water channels to prevent the whole device from being deformed due to excessive temperature.

The master cylinder 20 and the blank holder 21 are fixed on the upper support plate 19 by bolts, and provide power for the punch 16 and the punch blank holder 3 respectively; The lower support plate 23 is connected, and the lower end of the strut 24 is connected with the base 25 through threads;

The central position of the base 25 is also provided with an infrared thermal imager 28 for real-time acquisition and recording of the temperature field distribution during the deformation process of the test sample; both sides of the infrared thermal imager 28 are provided with a light source 26 and a high-frequency The camera 27 is used to acquire and record the strain field distribution of the test sample in real time.

A circular through hole 29 is provided in the middle of the lower support plate 23, and the through hole 29 serves as a window for collecting the temperature field and the strain field.

Rounded corners are provided at the intersection of the side surface of the male die pressing ring 3 and the female die pressing ring 4 in contact with the test sample and the inner wall. A circular draw bead 8 is provided on the side of the punch rim 3 in contact with the test sample, and the draw bead 8 matches the groove 6 on the female die rim 4 to prevent the test sample There are rounded corners at the junction of the draw bead 8 and the surface of the punch rim 3 ; rounded corners are set at the junction of the groove 6 and the surface of the die rim 4 .

As shown in FIG. 8 , the present invention discloses a method for obtaining the differential temperature forming limit of a variable-strength steel plate by using the above-mentioned test device, including the following steps:

S1. Prepare a sample, spray the surface of the test sample with black and white high temperature resistant speckles, riveted a thermocouple on the edge of the test sample, place the test sample in a heating furnace to heat up to a specified temperature, and keep it for a period of time;

S2. Coat the punch with high temperature resistant grease, and press the edge ring 3 of the punch down to touch the edge ring 4 of the concave die to reduce the heat loss during the heating process;

S3, turn on the high-frequency induction heating coil 13, rapidly heat the blank holder to the test temperature, and collect the temperature of the blank holder by the infrared probe 22, and realize closed-loop control with the high-frequency induction coil 13; turn on the heating on both sides of the punch 16 rod, or only turn on one side of the heating rod, by adjusting the power of the heating rod, the two sides of the punch 16 have different temperatures. The temperature of the punch 16 is collected by the thermocouples on both sides and realizes closed-loop control. The specific temperature is based on the required forming strength. Depending on the requirements of the steel plate;

S4, turn on the light source 26, the high-frequency camera 27 and the infrared thermal imager 28;

S5. After the test sample is heated to a specified temperature and kept warm, the punch rim 3 goes up until the test sample can be accommodated to enter the gap between the punch rim 3 and the female die rim 4 ; quickly transfer the heated test sample out of the heating furnace through the mechanical arm, and place it at the designated position on the die pressing ring 4;

S6. Quickly close the mold, and the punch rim 3 presses the test sample on the female die rim 4;

S7. The speed of the punch 16 is controlled in sections. The punch 16 first descends rapidly until it is about to contact the test sample, and the speed is adjusted to the set test speed; the punch 16 punches the test sample until the test sample appears strong At the same time, the temperature field distribution of the test sample is recorded in real time by the infrared thermal imager 28, and the strain field distribution of the test sample is recorded in real time by the high frequency camera 27;

S8. Extract the strain value at the critical rupture moment, and obtain the ultimate strain at the rupture by interpolation method;

S9, the punch blank holder 3 and the punch 16 go up, the blank holder ring is opened, and the sample after the test is taken out;

S10. Repeat steps S1-S9 to perform forming limit tests under different strain paths, temperatures, strain rates, etc., to establish differential temperature forming limit curves of the variable-strength steel plate under different test conditions.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, but not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: The technical solutions described in the foregoing embodiments can still be modified, or some or all of the technical features thereof can be equivalently replaced; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the technical solutions of the embodiments of the present invention. scope.

Claims (7)

1. A test device for obtaining the differential temperature forming limit of a variable-strength steel plate is characterized by comprising:

the punch (16) comprises two punch main bodies (1) with symmetrical structures, an insert (5) and a ceramic heat insulation layer (2); the ceramic heat insulation layer (2) is arranged between the two punch head main bodies (1) in a symmetrical structure and is used for isolating heat transfer at two sides of the punch head (16); the punch head main body (1) and the ceramic heat insulation layer (2) are tightly connected together through bolts; the insert (5) is fixed at the bottom of the punch (16) through a bolt and forms a smooth-transition spherical surface with the punch body (1); a deep hole for installing a heating rod and a thermocouple is further formed in the punch (16); the top of the punch (16) is provided with a punch top threaded hole (10) for connecting a connecting rod (15), and the punch (16) is connected with a main cylinder (20) through the connecting rod (15);

the blank holder is embedded and arranged on the outer side of the punch (16) in an up-down mode and is divided into a male die blank holder (3) and a female die blank holder (4), a high-frequency induction heating coil (13) and an infrared probe (22) are arranged around the male die blank holder (3) and the female die blank holder (4) respectively, and the infrared probe (22) and the high-frequency induction heating coil (13) form a closed-loop control system for rapidly heating the blank holder and keeping the blank holder at a specified test temperature;

a support structure comprising a base (25), a pillar (24) fixed on the base (25), and a support body for fixing the punch (16); the supporting main body comprises an upper supporting plate (19), a guide post (18), a sliding block (17) and a lower supporting plate (23); the female die blank holder (4) is fixed on the lower supporting plate (23) through a bolt, and the male die blank holder (3) is fixed on the sliding block (17) through a bolt; the slide block (17) is connected with a blank pressing cylinder (21) to drive the male die blank pressing ring (3) to move up and down along the guide pillar (18); the upper end of the guide post (18) is connected with the upper supporting plate (19) through threads, and the lower end of the guide post (18) is fixed on the lower supporting plate (23) through threads; the main cylinder (20) and the blank pressing cylinder (21) are fixed on the upper supporting plate (19) through bolts and respectively provide power for the punch (16) and the male die blank holder (3); the upper end of the strut (24) is connected with the lower supporting plate (23) through threads, and the lower end of the strut (24) is connected with the base (25) through threads;

the middle part of the base (25) is also provided with a thermal infrared imager (28) which is used for acquiring and recording the temperature field distribution of the test sample in the deformation process in real time; and a light source (26) and a high-frequency camera (27) are arranged on two sides of the thermal infrared imager (28) and are used for acquiring and recording strain field distribution of the test sample in real time.

2. The test device for obtaining the differential forming limit of the variable-strength steel plate according to claim 1, wherein an asbestos pad (14) is provided at a junction of the connecting rod (15) and the punch (16) for preventing heat of the punch (16) from being transferred to the master cylinder (20) through the connecting rod (15).

3. The test device for obtaining the differential forming limit of the variable-strength steel plate according to claim 1, wherein cooling water channels are arranged inside the upper support plate (19) and the lower support plate (23) for preventing the deformation of the whole device due to the overhigh temperature.

4. The test device for acquiring the differential temperature forming limit of the variable-strength steel plate as claimed in claim 1, wherein a circular through hole (29) is formed in the middle of the lower support plate (23), and the through hole (29) is used as a window for collecting a temperature field and a strain field.

5. The test device for obtaining the differential forming limit of the variable-strength steel plate according to claim 1, wherein the intersections of the inner wall and one side surface of the male die blank holder (3) and one side surface of the female die blank holder (4) which are in contact with the test sample are provided with round corners.

6. The test device for obtaining the differential forming limit of the variable-strength steel plate according to claim 1, wherein a circular draw bead (8) is arranged on one side of the male die blank holder (3) which is in contact with the test sample, and the draw bead (8) is matched with a groove (6) on the female die blank holder (4) and used for preventing the test sample from flowing; a fillet is arranged at the junction of the draw bead (8) and the surface of the male die blank holder (3); and a fillet is arranged at the junction of the surface of the groove (6) and the surface of the female die blank holder (4).

7. A method for obtaining the differential temperature forming limit of a variable strength steel plate by using the test device of any one of claims 1 to 6, which is characterized by comprising the following steps:

s1, preparing a sample, spraying black and white high-temperature resistant speckles on the surface of the sample, riveting a thermocouple at the edge of the sample, placing the sample in a heating furnace, heating to a specified temperature, and keeping the temperature for a period of time;

s2, coating high-temperature-resistant lubricating grease on the punch, enabling the male die blank holder (3) to move downwards, and pressing the female die blank holder (4) in a contact manner to reduce heat loss in the heating process;

s3, starting the high-frequency induction heating coil (13), rapidly heating the blank holder to a test temperature, collecting the temperature of the blank holder through an infrared probe (22), and realizing closed-loop control with the high-frequency induction heating coil (13); starting heating rods on two sides of the punch (16), or starting only one heating rod on one side, enabling the two sides of the punch (16) to have different temperatures by adjusting the power of the heating rods, and acquiring the temperature of the punch (16) through thermocouples on the two sides and realizing closed-loop control;

s4, turning on a light source (26), a high-frequency camera (27) and a thermal infrared imager (28);

s5, after the sample to be tested is heated to the specified temperature and is subjected to heat preservation, the male die blank holder (3) moves upwards until the sample to be tested can be accommodated into a gap between the male die blank holder (3) and the female die blank holder (4); the heated test sample piece is rapidly transferred out of the heating furnace through a mechanical arm and placed at a specified position on the female die blank holder (4);

s6, rapidly closing the die, wherein the male die blank holder (3) tightly presses the test sample on the female die blank holder (4);

s7, controlling the speed of the punch (16) in a segmented mode, and quickly descending the punch (16) until the punch is in contact with a test sample piece, wherein the speed is adjusted to be a set test speed; the punch (16) punches the test sample piece until the test sample piece shrinks or cracks; simultaneously, recording the temperature field distribution of the test sample piece in real time by an infrared thermal imager (28), and recording the strain field distribution of the test sample piece in real time by a high-frequency camera (27);

s8, extracting a strain value at the critical fracture moment, and obtaining the limit strain at the fracture part by an interpolation method;

s9, moving the male die blank holder (3) and the punch (16) upwards, opening the blank holder, and taking out a sample after the test;

and S10, repeating the steps S1-S9, performing forming limit tests under different strain paths, temperatures and strain rates, and establishing differential temperature forming limit curves of the variable strength steel plate under different test conditions.

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