CN108279175B - Test device and method for obtaining differential temperature forming limit of variable-strength steel plate - Google Patents
Test device and method for obtaining differential temperature forming limit of variable-strength steel plate Download PDFInfo
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- CN108279175B CN108279175B CN201810204624.3A CN201810204624A CN108279175B CN 108279175 B CN108279175 B CN 108279175B CN 201810204624 A CN201810204624 A CN 201810204624A CN 108279175 B CN108279175 B CN 108279175B
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N3/30—Investigating strength properties of solid materials by application of mechanical stress by applying a single impulsive force, e.g. by falling weight
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/84—Systems specially adapted for particular applications
Abstract
The invention provides a test device for obtaining the differential temperature forming limit of a variable-strength steel plate, which comprises a punch head, a punch head and a ceramic heat-insulating layer, wherein the punch head consists of a punch head main body, an insert block and a ceramic heat-insulating layer; the ceramic heat insulation layer is arranged in the middle of the punch main body and fixedly connected with the punch main body; the insert is fixed at the bottom of the punch and forms a smooth-transition spherical surface with the punch body; a deep hole for installing a heating rod and a thermocouple is formed in the punch; the top of the punch is provided with a threaded hole for connecting a connecting rod; a high-frequency induction heating coil and an infrared probe are respectively arranged around the convex die blank holder and the concave die blank holder to form a closed-loop control system; the device also comprises an upper supporting plate, a lower supporting plate, a sliding block, a guide pillar, a support column, a base, a main cylinder, a blank pressing cylinder, a light source, a high-frequency camera and a thermal infrared imager. According to the invention, the temperatures of the two sides of the punch are controlled, so that the high-temperature test sample piece has different quenching rates on the two sides of the punch, thereby simulating the differential temperature forming process of the variable-strength steel plate, and acquiring the differential temperature forming limit of the variable-strength steel plate through a strain field acquired by a high-frequency camera.
Description
Technical Field
The invention relates to the field of hot forming of high-strength steel plates, in particular to a test device and a test method for obtaining a differential temperature forming limit of a variable-strength steel plate.
Background
The rapid development of the automobile industry greatly improves the traveling efficiency of people, promotes the social development, and brings many safety problems and environmental problems. The design of light weight is realized on the basis of ensuring safety, the fuel consumption and the exhaust emission of the automobile can be effectively reduced, and the design is a necessary trend for the future development of the automobile. Optimizing the design of automotive structures, using new materials and corresponding advanced manufacturing techniques are the main methods to achieve lightweight designs.
The hot forming technology of ultrahigh strength steel plate is an advanced manufacturing technology specially used for forming high strength steel plate, and the general process is that the steel plate is heated to over 900 ℃ to be fully austenitized, then is rapidly stamped and formed, and is quenched under pressure to obtain a high strength member with uniform martensite structure. The ultrahigh-strength steel hot forming technology can be used for obtaining parts with the strength exceeding 1500MPa, so that the thickness of the component can be properly reduced under the condition of ensuring that the strength of the parts is not reduced, and the aim of lightening the automobile is fulfilled.
For some safety components of automobiles, such as B-pillars, inner door panels, etc., the components are required to have sufficient strength to prevent the automobile structure from being seriously damaged, and some parts of the components are required to have lower strength and higher energy absorption property, so that the overall collision safety of the automobile is improved. The traditional hot forming technology can only obtain ultrahigh-strength steel components with single performance, and can not meet the requirements of different mechanical properties required by different parts of the components; by changing the quenching rate of different parts of the component in the forming process, the variable-strength structural component with different microstructures and mechanical properties can be obtained, and the variable-strength structural component can be applied to a vehicle body to improve the overall collision safety of the vehicle to the greatest extent.
For formability evaluation of hot formed panels, the most common and intuitive method applied is the forming limit diagram (FLD, or FLC). The forming limit diagram is a curve formed by a primary strain coordinate system and a secondary strain coordinate system at strain points of materials beginning to shrink under different (approximate) linear strain paths in a forming process. For example, chinese patent (application number 201410076406.8) discloses a device and a method for testing the limit of hot forming of a metal plate, which can prevent the heat transfer between a sheet and a punch during a test by heating the sheet and the punch in a sealed box, thereby reducing the test error. However, the forming of actual strength-variable steel sheets is a non-isothermal process, with the temperature being continuously varied; meanwhile, different positions of the variable-strength steel plate have different quenching rates, and the variable-strength steel plate is a differential temperature forming process, so that the differential temperature forming limit of the variable-strength steel plate cannot be obtained by the device and the method. Chinese patent (application number 201210192708.2) discloses a numerical simulation prediction method for transient forming limit of ultra-high strength steel hot forming, which is characterized in that after a material high-temperature uniaxial tensile experiment is carried out to obtain constitutive characteristic parameters of a hot forming plate, an M-K model for predicting the hot forming limit under a steady state condition is established by using an M-K theory, and the forming limit under different hot forming transient process conditions is calculated and predicted based on the determined model, but a specific test result is lacked for verification.
In summary, at present, there is no relevant report on a testing apparatus and method for a differential temperature forming process of a variable strength steel plate, and a special testing apparatus and method are urgently needed to realize the differential temperature forming process of the variable strength steel plate, test the forming limit of the variable strength steel plate and evaluate the forming performance of the variable strength steel plate.
Disclosure of Invention
In view of the technical problems of the conventional forming limit test equipment and method, the test device and method for obtaining the differential temperature forming limit of the variable-strength steel plate are provided. The test device provided by the invention can simulate the forming process of the variable-strength steel plate to obtain the variable-strength component with specific microstructure and mechanical property distribution; meanwhile, the testing device can acquire a strain field in the steel plate forming process in real time through an integrated optical strain measurement system, and an accurate variable-strength steel plate forming limit diagram is established.
The technical means adopted by the invention are as follows:
a test device and a method for obtaining the differential temperature forming limit of a variable-strength steel plate are characterized by comprising the following steps:
the punch mainly comprises two punch main bodies with symmetrical structures, an insert and a ceramic heat insulation layer; the ceramic heat insulation layer is arranged between the two punch head main body symmetrical structures and is used for isolating heat transfer at two sides of the punch head; the punch head main body and the ceramic heat insulation layer are tightly connected together through bolts; the insert is fixed at the bottom of the punch through a bolt, forms a smooth-transition spherical surface with the punch main body, and plays a role in protecting a ceramic heat-insulating layer; the punch is internally provided with a deep hole for mounting a heating rod and a thermocouple; the top of the punch is provided with a punch top threaded hole for connecting a connecting rod, and the punch is connected with the main cylinder through the connecting rod;
the blank holder is embedded and arranged on the outer side of the punch and is divided into a male die blank holder and a female die blank holder, a high-frequency induction heating coil and an infrared probe are respectively arranged around the male die blank holder and the female die blank holder, and the infrared probe and the high-frequency induction heating coil form a closed-loop control system for rapidly heating the blank holder and keeping the blank holder at a specified test temperature;
the supporting structure mainly comprises a base, a support column fixed on the base and a supporting main body used for fixing the punch; the supporting main body comprises an upper supporting plate, a guide pillar, a sliding block and a lower supporting plate; the female die blank holder is fixed on the lower supporting plate through a bolt, and the male die blank holder is fixed on the sliding block through a bolt; the slide block is connected with the blank pressing cylinder to drive the male die blank pressing ring to move up and down along the guide pillar; the upper end of the guide post is connected with the upper supporting plate through threads, and the lower end of the guide post is fixed on the lower supporting plate through threads; the main cylinder and the blank pressing cylinder are fixed on the upper supporting plate through bolts and respectively provide power for the punch and the male die blank holder; the upper end of the strut is connected with the lower supporting plate through threads, and the lower end of the strut is connected with the base through threads;
the middle part of the base is also provided with a thermal infrared imager for acquiring and recording the temperature field distribution of the test sample in the deformation process in real time; and light sources and high-frequency cameras are arranged on two sides of the thermal infrared imager and are used for acquiring and recording strain field distribution of the test sample piece in real time.
Further, an asbestos pad is arranged at the joint of the connecting rod and the punch and used for preventing heat of the punch from being transmitted to the main cylinder through the connecting rod.
Furthermore, the upper supporting plate and the lower supporting plate are internally provided with cooling water channels for preventing the device from being deformed due to overhigh temperature.
Furthermore, the middle part of lower support plate is equipped with circular through-hole, the through-hole is as the window of gathering temperature field and strain field.
Furthermore, fillets are arranged at the intersection of the inner wall and the surface of one side, in contact with the test sample, of the male die blank holder and the female die blank holder.
Furthermore, a circular draw bead is arranged on one side, which is in contact with the test sample piece, of the convex die blank holder, and the draw bead is matched with a groove in the concave die blank holder and used for preventing the test sample piece from flowing; a fillet is arranged at the junction of the draw bead and the surface of the male die blank holder; and a fillet is arranged at the junction of the groove and the surface of the female die blank holder.
The invention discloses a method for acquiring the differential temperature forming limit of a variable-strength steel plate by applying the test device, which is characterized by comprising the following steps of:
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 to move downwards, and touching and pressing the female die blank holder to reduce heat loss in the heating process;
s3, starting the high-frequency induction heating coil, rapidly heating the blank holder to a test temperature, collecting the temperature of the blank holder through an infrared probe, and realizing closed-loop control with the high-frequency induction coil; starting heating rods on two sides of the punch or starting the heating rod on one side, enabling the two sides of the punch to have different temperatures by adjusting the power of the heating rods, and acquiring the temperature of the punch through thermocouples on the two sides and realizing closed-loop control;
s4, turning on a light source, a high-frequency camera and a thermal infrared imager;
s5, after the sample to be tested is heated to the specified temperature and is subjected to heat preservation, the male die blank holder ascends until the sample to be tested can be accommodated into a gap between the male die blank holder and the female die blank holder; rapidly transferring the heated test sample out of the heating furnace through a mechanical arm, and placing the test sample at a specified position on the female die blank holder;
s6, rapidly closing the die, wherein the male die blank holder tightly presses the test sample piece on the female die blank holder;
s7, controlling the speed of the punch in a segmented mode, and quickly descending the punch until the punch is in contact with a test sample piece, wherein the speed is adjusted to be a set test speed; punching the test sample piece by using the punch until the test sample piece is shrunk or broken; simultaneously, recording the temperature field distribution of the test sample piece in real time by the thermal infrared imager, and recording the strain field distribution of the test sample piece in real time by the high-frequency camera;
s8, extracting a strain value at the critical fracture moment, and obtaining the limit strain at the fracture part by an interpolation method;
s9, enabling the male die blank holder and the punch to ascend, opening the blank holder, and taking out a sample piece after the test;
and S10, repeating the steps S1-S9, performing forming limit tests under different conditions of strain paths, temperatures, strain rates and the like, and establishing differential temperature forming limit curves of the variable strength steel plate under different test conditions.
The invention has the following advantages:
1) according to the test device, two sides of the punch can be heated to different temperatures, and after the punch contacts the sample, two sides of the sample can have different quenching rates, so that the forming process of the variable-strength steel plate is simulated;
2) the testing device heats the blank holder to the same temperature as the sample piece, and prevents the sample piece from exchanging heat with the blank holder after being pressed, so that the testing precision is not influenced;
3) the testing device disclosed by the invention adopts the high-frequency induction coil to heat and insulate the blank holder, so that the heating efficiency is high, the temperature distribution is uniform, and the heat insulation effect is good;
4) the test device adopts the high-frequency camera to acquire and record the strain field distribution and the strain value at the critical rupture moment in the deformation process of the sample piece in real time, and is convenient, quick and high in accuracy;
5) the forming limit test which can be carried out by the test device is not only limited to a variable-strength steel plate, but also can be carried out by a common non-isothermal forming limit test, and at the moment, the temperature of two sides of the punch is only required to be set to be the same;
6) the testing device can also be used for carrying out isothermal forming limit tests within a certain temperature range, and at the moment, the punch, the blank holder and the sample piece only need to be heated to the same temperature.
For the above reasons, the present invention can be widely applied to the field of hot forming of high-strength steel sheets.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed to be used in the description of the embodiments or the prior art will be briefly introduced below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to these drawings without creative efforts.
FIG. 1 is a central longitudinal sectional view of the punch, the male die blank holder and the female die blank holder of the present invention.
Fig. 2 is a front view of the punch of the present invention.
Fig. 3 is a top view of the punch of the present invention.
Fig. 4 is a left side view of the punch of the present invention.
Fig. 5 is a sectional view taken along the line a-a in fig. 3.
Fig. 6 is a schematic view of the punch assembly of the present invention.
FIG. 7 is a schematic structural diagram of the testing apparatus of the present invention.
FIG. 8 is a flow chart of the test method of the present invention.
In the figure: 1. a punch body; 2. a ceramic thermal insulation layer; 3. a male die blank holder; 4. a female die blank holder; 5. an insert; 6. a groove; 7. an insert threaded hole; 8. a draw bead; 9. a symmetrical threaded hole; 10. a threaded hole is formed in the top of the punch; 11. a heating rod mounting hole; 12. a thermocouple mounting hole; 13. a high-frequency induction heating coil; 14. an asbestos pad; 15. a connecting rod; 16. a punch; 17. a slider; 18. a guide post; 19. an upper support plate; 20. a master cylinder; 21. a blank pressing cylinder; 22. an infrared probe; 23. a lower support plate; 24. a pillar; 25. a base, 26, a light source; 27. a high frequency camera; 28. provided is a thermal infrared imager.
Detailed Description
In order to make the objects, 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 drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
As shown in fig. 7, a test apparatus and method for obtaining a differential forming limit of a strength-variable steel sheet includes:
the punch 16 (shown in fig. 1-5) mainly comprises two punch bodies 1 with symmetrical structures, an insert 5 and a ceramic heat insulation layer 2; the ceramic heat insulation layer 5 is arranged between the two symmetrical structures of the punch head main body 1 and is used for isolating heat transfer at two sides of the punch head 16; the punch body 1 and the ceramic heat insulation layer 5 are tightly connected together by bolts passing through symmetrical threaded holes 9 on one side of the punch body 1; the insert 5 penetrates through an insert threaded hole 7 through a bolt and is fixed at the bottom of the punch 16, and forms a smooth-transition spherical surface with the punch main body 1, and meanwhile, the insert plays a role in protecting the ceramic heat insulation layer 2; a heating rod mounting hole 11 for mounting a heating rod and a thermocouple mounting hole 12 for mounting a thermocouple are 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; an asbestos pad 14 is disposed at the junction of the connecting rod 15 and the punch 16 to prevent heat of the punch 16 from being transferred to the master cylinder 20 through the connecting rod 15.
The blank holder (as shown in fig. 6) is embedded and sleeved outside the punch 16 and is arranged up and down, 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 respectively arranged around the male die blank holder 3 and the female die blank holder 4, 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;
the supporting structure mainly comprises a base 25, a support column 24 fixed on the base 25 and a supporting body for fixing the punch 16; the supporting 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 6 bolts, and the male die blank holder 3 is fixed on the sliding block 17 through 6 bolts; the slide block 17 is connected with a blank holder cylinder 21 to drive the male die blank holder 3 to move up and down along the guide pillar 18; the upper end of the guide post 18 is connected with the upper support plate 19 through threads, and the lower end of the guide post 18 is fixed on the lower support plate 23 through threads; and cooling water channels are arranged in the upper supporting plate 19 and the lower supporting plate 23 and are used for preventing the device from being deformed due to overhigh temperature.
The main cylinder 20 and the blank holder cylinder 21 are fixed on the upper support 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 central position of the base 25 is also provided with a thermal infrared imager 28 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 the strain field distribution of the test sample in real time.
The middle part of the lower supporting plate 23 is provided with a circular through hole 29, and the through hole 29 is used as a window for collecting a temperature field and a strain field.
And fillets are arranged at the intersection of the inner wall and the surface of one side, which is in contact with the test sample, of the male die blank holder 3 and the female die blank holder 4. A round draw bead 8 is arranged on one side of the male die blank holder 3, which is in contact with the test sample piece, and the draw bead 8 is matched with the groove 6 on the female die blank holder 4 and used for preventing the test sample piece 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 groove 6 and the surface of the female die blank holder 4.
As shown in FIG. 8, the invention discloses a method for obtaining the differential temperature forming limit of a variable-strength steel plate by using the test device, which comprises 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 touching and pressing the female die blank holder 4 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 blank holder temperature through the infrared probe 22, and realizing closed-loop control with the high-frequency induction coil 13; starting heating rods on two sides of the punch 16, or starting only one heating rod, enabling the two sides of the punch 16 to have different temperatures by adjusting the power of the heating rods, acquiring the temperature of the punch 16 through thermocouples on the two sides, and realizing closed-loop control, wherein the specific temperature is determined according to the requirement of forming the variable-strength steel plate;
s4, turning on the light source 26, the high-frequency camera 27 and the 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; rapidly transferring the heated test sample out of the heating furnace by a mechanical arm, and placing the test sample 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 piece 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; meanwhile, the thermal infrared imager 28 records the temperature field distribution of the test sample piece in real time, and the high-frequency camera 27 records the strain field distribution of the test sample piece in real time;
s8, extracting a strain value at the critical fracture moment, and obtaining the ultimate 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 the sample after the test;
and S10, repeating the steps S1-S9, performing forming limit tests under different conditions of strain paths, temperatures, strain rates and the like, and establishing 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 solution of the present invention, and not to limit the same; while the invention has been described in detail and with reference to the foregoing embodiments, it will be understood by those skilled in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.
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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