SYSTEM AND METHOD OF QUENCHING MONITORING DURING STEEL HOT STAMPING

Abstract

 A system and method of quenching monitoring during steel hot stamping is presented, based on real-time measurements of pressure variations caused by phase transformations from austenite to martensite states, which undergo during quenching. The system comprises stamping means (300) with at least one pressing tool (320) which forms and quench a steel sheet (400); at least one pressure sensor (100) with at least a force gauge (110) which measures a time-resolved pressure curve (600) generated by volume alterations of the steel sheet (400) during quenching; and a processor (200) which determines when quenching is finalized by detecting an overshot (630) in the measured time-resolved pressure curve (600). The at least one pressure sensor (100) may either be located in a complementary sensor receptacle (310) of the stamping means (300), in contact with a slowest-cooling region (420) of said steel sheet (400); or in an auxiliary structure (500) which is in contact with a removable protrusion (430) of the steel sheet (400) which emulates quenching properties of said slowest-cooling region (420).

Description

FIELD OF THE INVENTION

[0001] The present invention has its application within the metallurgy sector, and especially, in the industrial area engaged in providing steel elements through hot stamping techniques.

BACKGROUND OF THE INVENTION

[0002] In hot stamping techniques (also referred to as press hardening, hot forming, hot pressing, die quenching and press quenching), a quenchable austenitized steel sheet undergoes both forming and quenching processes in the same die and a with a single stroke of the pressing tools. The technique employed for cooling the austenitized steel sheet may vary depending on the particular hot stamping technology. For example, WO 2018/053669 A1 discloses a cast steel die which incorporates cooling pipes, whereas WO 2018/055918 A1 employs a plurality of nozzles which spray cooling water directly on the metal surface. However, regardless of the particular cooling technique being employed, ensuring that the quenching is successfully performed in all the required areas of the steel sheet is a critical issue for this technology, since the proper completion of the quenching process is required in order to guarantee that the formed steel sheet achieves the required mechanical properties.

[0003] Currently, three main strategies are applied in order to guarantee the completion of the quenching process during hot stamping of metal sheets. The first strategy is as simple as employing a quenching time which greatly exceeds the minimum time estimated to ensure proper quenching. That is, the steel sheet is left inside the pressing tools for a long period of time, thereby granting that the quenching process finalizes long before the formed steel sheet removal. Of course, this first approach presents the inherent problem of an unoptimized quenching time, which results in longer fabrication periods and a decreased productivity. Furthermore, this first strategy is unable to identify in real time any problem associated to the raw material that may be affecting the quenching process, such as incomplete austenitizing incidences. For example, in the particular case of 22MnB5 steels, solute Boron may deplete due to combination of Boron with Nitrogen into Boron Nitride, rendering invalid the original quenching time estimation.

[0004] The second strategy is based on numerical computations performed by specific simulation software, which calculate the necessary quenching time through estimations of the heat transfer and material properties of a particular scenario. A first example of simulation methods for analyzing quenching processes is disclosed in JP 2010/230331 A, for the particular case of high-frequency quenching. In this first example, a finite element model (FEM) providing electromagnetic field analysis is employed in combination with another FEM model which characterizes heat-transfer elements. In JP 2001/049333 A, the accuracy of the numerical computation is enhanced by incorporating a critical cooling rate experimentally measured on material tests. Finally, WO 2016/208204 A1 presents a heating simulation method, which in this case is particularized to welding regions in the peripheral regions of steel sheets.

[0005] Regardless of the particular computation method which is applied to model the steel sheet behavior during quenching, the additional knowledge of the material dynamics provided by this second strategy enables a much greater degree of quenching time optimization than the first alternative. However, this second approach lacks any real-time monitoring capable of detecting any of the aforementioned problem associated to the raw material that may be affecting the quenching process. Furthermore, as the computational models are based on ideal geometry, pressure and temperature values, any deviations of these parameters between the model and the actual application scenario, may result in an erroneous estimation of the required quenching time.

[0006] Finally, the third strategy is based on an on-line non-destructive control system, such as ultrasonic contact impedance, which is used to check if quenching was correct right after the part is extracted from the press. This technique enables to determine if the quenching was correct after removal from the die, hence allowing to make corrections such as increasing the quenching time if the results of the measurements were not satisfactory.

[0007] However, this information is provided only after the hot stamping process is finalized. Therefore, the steel sheets generating unsatisfactory tests cannot be salvaged. Moreover, depending the geometry of the part being stamped, this method may require many adjustments until the quenching operation is completely achieved in the whole required areas of the part, and real-time monitoring to detect unexpected problems or modifications in the quenching process is still not provided.

[0008] Therefore, there is still the need in the state of the art of a non-invasive quenching monitoring technique for steel hot stamping processes, capable of providing real-time information of the material processes taking place within the steel sheet, hence enabling to optimize quenching time, guaranteeing final mechanical properties of the formed items, and adapting to unexpected changes or problems occurring during the hot stamping process.

SUMMARY OF THE INVENTION

[0009] The current invention solves all the aforementioned problems by disclosing a monitoring technique capable of determining in real time the completion of quenching processes during steel hot stamping. This is achieved by measuring and analyzing time-resolved pressure variations caused by phase transformations from austenite to martensite states, which occur during said quenching process.

[0010] In a first aspect of the present invention, a system of quenching monitoring during steel hot stamping according to claim 1 is disclosed. Preferred embodiments of the invention are defined in dependent claims.

[0011] The system comprises at least the following elements:

• Stamping means with one or more pressing tools, adapted to form and quench steel sheets. That is, during system operation, a main stamping body of a heated steel sheet is pressed in a desired shape defined by the pressing tools geometry. The stamping means not only generated said desired shape, but also extract heat from the steel sheet, effectively quenching the steel and determining its mechanical properties. Notice that during the quenching process, the phase changes inside the steel sheet, namely from an austenite state to a martensite state, results in a volume variation traditionally ignored or automatically compensated through feedback loops by conventional hot stamping techniques. Since the volume variations can be more apparent in certain pressing conditions (i.e. in a range of forces applied during pressing), the stamping means are preferably configured to apply said pressing conditions for optimal monitoring.
• At least one pressure sensor (with one or more force gauges) configured to measure pressure alterations generated by the aforementioned volume variation of the austenite to martensite phases. That is, the at least one pressure sensor provides a time-resolved pressure curve whose analysis enables monitoring the quenching process. In order to provide said time-resolved pressure curve, the pressure sensor may preferably either be in direct contact with a flat surface the steel sheet, or connected to said steel sheet through a force transmitting element, such as a rod or a punch. Since the quenching times of different areas of the steel sheet may vary depending on the sheet geometry and on the disposition of cooling elements of the stamping means, the pressure sensor is preferably disposed either in the vicinity of the slowest-cooling region of the steel sheet, or in a reference area whose response is calibrated against said slowest-cooling region. Furthermore, notice that multiple pressure sensors near multiple areas of the steel sheets may be employed.
• A processor, which receives the time-resolved pressure curve measured by the pressure sensor (or sensors), and determines when the quenching process is finalized based on an overshot in said time-resolved pressure curve. That is, the processor determines the presence and/or the stabilization of a pressure increase (i.e. pressure overshot) beyond a first threshold, where said first threshold is reached by the regular operation of the press tools, and the overshot is caused by steel sheet volume expansion.

[0012] Note that the monitoring performed by the processor is of local nature, that is, a single pressure curve reflects the state of the quenching process in the area of the steel sheet surrounding the pressure sensor. In order to guarantee that the full steel piece is properly formed and quenched, the process finalization is only determined when all areas to be quenched have undergone the phase transformation between austenite and martensite states, or in an equivalent manner, when the slowest-cooling region has undergone said transformation.

[0013] Nevertheless, note that some hot stamping designs account for a partial quenching, that is, it is not required for all parts of the product leaving the stamping means to be fully quenched. Therefore, whenever the terms "fully quenched" or "slowest-cooling region" are used in the description, they should be understood as excluding those parts which aren't required to be quenched by design.

[0014] Also note that, in particular embodiments of the invention, the processor and the at least one pressure sensor may be integrated in a joint device. That is, the analysis of the time-resolved pressure curve and the subsequent overshot detection may be performed in data processing means directly connected to the force gauge, such as a digital signal processor, a field-programmable gate array, an application-specific integrated circuit, a micro-processor, a micro-controller, or any other form of programmable hardware. In this case, when quenching finalization is detected at said joint device, a trigger signal or notification is preferably transmitted to the user interface or to an automated actuator of the stamping means, circumventing the need of transmitting the complete time-resolved pressure curve data.

[0015] There are two preferred alternatives regarding the location of the pressure sensor respective to the stamping means. In a first preferred alternative, the pressure sensor (or at least one of multiple pressure sensors) is attached to the stamping means, preferably by introducing mechanical attaching means of said pressure sensor (such as threads, screws, etc) into a complementary receptacle located in said stamping means. More preferably, the location of the receptacle wherein the sensor is attached is previously selected in order to maintain sensor proximity with the slowest-cooling region of the steel sheet. Since the sensor is integrated in the stamping means, a wireless communication between the sensor and the processor is preferred for this alternative.

[0016] Furthermore, the slowest-cooling region is defined by the stamping means design (mainly by the cooling means location and the geometry that said stamping means generate in the steel sheets), and its position remains unchanged for any number of stamped steel sheets with the same stamping means design. Therefore, the position of the slowest-cooling region can be determined previously to the monitoring system operation. Said position determination may be performed, for example, through numerical simulations, through theoretical geometry analysis, and/or through experimental testing.

[0017] In a second preferred alternative for the pressure sensor location, the system comprises an auxiliary structure which, during hot stamping of a steel sheet, remains in contact with a removable protrusion of the steel sheet. That is, steel sheets provided to the system comprise a lateral extension for monitoring purposes, which is removed from the final product in a later stage. The auxiliary structure may either be directly connected to the stamping means or be an independent structure with separate surfaces and pressing tools.

[0018] Furthermore, the auxiliary structure may comprise any number of calibration means in order to reproduce in the removable protrusion the same quenching behaviour as in the slowest-cooling region of the main stamping body of the steel sheet. That is, since the geometry of the removable protrusion, and its disposition respective to the stamping means, may be significantly differ from those of the slowest-cooling region, the auxiliary structure is preferably adapted to induce a force and cooling conditions that guarantee that complete quenching is achieved simultaneously (or almost simultaneously) in the removable protrusion and in the slowest-cooling region. This is achieved, preferably, by a hydraulic actuator, a flow control valve which controls a flow of cooling fluid shared with the stamping means, and/or any other calibration means that affect the quenching dynamics of the removable protrusion.

[0019] In a second aspect of the present invention, a method of quenching monitoring during a steel hot stamping process is disclosed. The method is applicable to hot stamping processes where at least one pressing tool both forms and quenches steel sheets. The phase transformation from austenite to martensite states inherently taking place during the steel cooling in this scenario is used to monitor the quenching process in real time, following the following steps:

• Preferably, determining a slowest-cooling region of the steel sheet under the same hot stamping conditions as those where the method is applied. This determination may preferably performed through numerical simulations, experimental testing, theoretical analysis or any combination thereof.
• Preferably, determining optimal hot stamping conditions, such as an optimal range of applied forces, where the visibility of the overshot associated to the finalization of the phase transformation from austenite to martensite states is maximized.
• Measuring a time-resolved pressure curve, whose variations are generated by the aforementioned volume alterations of the steel sheet during quenching. This step is preferably implemented by means of at least one pressure sensor which may either be located in an area near (or in contact with) the slowest-cooling region of the steel sheet or in an area near (or in contact with) a removable protrusion which emulates quenching properties of the slowest-cooling region.
• Determining when quenching is finalized by locating an overshot in the measured time-resolved pressure curve.

[0020] Note that elements of the system of the invention may be configured to implement any preferred option and/or particular embodiments of the method of the invention, and vice versa.

[0021] In a third aspect of the present invention, a computer program is disclosed which implements the method of the invention. The computer program comprises computer program code means adapted to perform any embodiment of the method of the invention when run on a computer, a digital signal processor, a field-programmable gate array, an application-specific integrated circuit, a micro-processor, a micro-controller, or any other form of programmable hardware.

[0022] With the disclosed system, method and computer program, real-time monitoring of quenching processes during steel hot stamping is enabled, hence minimizing the duration of steel cooling while guaranteeing that all areas of the steel sheet are properly cooled, and safeguarding the mechanical properties of the final product. Additional advantages and features of the invention will become apparent from the detailed description that follows and will be particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] For the purpose of aiding the understanding of the characteristics of the invention, according to a preferred practical embodiment thereof and in order to complement this description, the following figures are attached as an integral part thereof, having an illustrative and non-limiting character:

Figure 1 schematically shows the main elements of the quenching monitoring system of the invention, according to a first preferred embodiment of the invention where the pressure sensor is attached to the stamping means.

Figure 2 illustrates said first preferred embodiment of the system of the invention by means of an example of application to the hot stamping of a steel sheet.

Figure 3 schematically shows the main elements of the quenching monitoring system of the invention, according to a first preferred embodiment of the invention where the pressure sensor is attached to an auxiliary structure.

Figure 4 illustrates said second preferred embodiment of the system of the invention by means of an example of a steel sheet with a removable protrusion.

Figure 5 also illustrates said second preferred embodiment of the system of the invention during hot stamping of the steel sheet of the example, using an auxiliary structure for the pressure sensor disposition.

Figure 6 presents an example of a time-resolved pressure curve generated by volume alterations in a steel sheet, caused by phase transformations from austenite to martensite states, which undergo during quenching.

DETAILED DESCRIPTION OF THE INVENTION

[0024] The matters defined in this detailed description are provided to assist in a comprehensive understanding of the invention. Accordingly, those of ordinary skill in the art will recognize that variation changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. In particular, note that any particular embodiment or feature of the device of the invention may be applied to the method of the invention and vice versa. Also, description of well-known functions and elements are omitted for clarity and conciseness.

[0025] Note that in this text, the term "comprises" and its derivations (such as "comprising", etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements, steps, etc.

[0026] In the context of the present invention, the term "approximately" and terms of its family (such as "approximate", etc.) should be understood as indicating values very near to those which accompany the aforementioned term. That is to say, a deviation within reasonable limits from an exact value should be accepted, because a skilled person in the art will understand that such a deviation from the values indicated is inevitable due to measurement inaccuracies, etc. The same applies to the terms "about" and "around" and "substantially".

[0027] Finally, note that for the sake of simplicity, all detailed embodiments are described with a single pressure sensor. Nevertheless, the same design and operation principles may be applied to a greater number of pressure sensors, distributed among different positions of the system in order to simultaneously monitor several regions of the steel sheet being stamped, and/or provide redundancy measurements of a given region. In case quenching completion is detected at different instants for the different regions, the fabrication process will not be considered complete until the slowest-cooling region is determined to be fully quenched.

[0028] Figure 1 presents the main elements of a first preferred embodiment of the system of the invention, which implements the steps of a first preferred embodiment of the method of the invention. The first preferred embodiment is configured to measure the force increase due to the austenite to martensite phase-transformation on the slowest-cooling region (420) of the main stamping body (410) of the steel sheet (400) being quenched.

[0029] The system comprises one or more pressure sensors (100) physically attached to stamping means (300) and wirelessly connected to a processor (200). Each pressure sensor (100) comprises at least one force gauge (110), such as strain gauges, piezoelectric sensors or any other pressure sensing solution known in the state of the art. The force gauges (110) are configured to measure a time-resolved pressure curve (600) of a region of the main stamping body (410) of the steel sheet (400). The measured time-resolved pressure curve (600) is wirelessly transmitted to the processor (200) through first connection means (130), preferably implemented through bluetooth technology, although any alternative wireless communication platform and protocol known in the state of the art may be employed.

[0030] Furthermore, the pressure sensor (100) also comprises attaching means (120), such as threads, press-fit rubber rings, or any other mechanical solution known in the state of the art which provides firm yet removable attachment between the pressure sensor (100) and a sensor receptacle (310) of the stamping means (300). A battery (140) is also provided to power the electronic elements of the pressure sensor (100).

[0031] The pressure sensor (100) is preferably disposed in a position which provides direct contact with a flat surface of the steel sheet (400), although force transmitting elements such as rods or punches may be used to connect the pressure sensor (100) and the steel sheet (400) instead. Although the required sensibility for accurately measuring the time-resolved pressure curve (600) may vary depending on the particular topology, size and configuration of the pressure sensor (100), it is advisable to use a pressure sensor (100) with a sampling rate over 1 Hz to properly detect the different stages of said time-resolved pressure curve (600) and optimize quenching time.

[0032] The stamping means (300), in which the pressure sensor (100) is integrated may be implemented with any type or configuration of pressing tools (320) known in the state of the art for hot stamping technology, as long as they are formally adapted to include a sensor receptacle (310) where the attaching means (120) of the pressure sensor (100) can be attached. Regardless of their particular technology, the stamping means perform two tasks: forming the steel sheet (400) into the desired shape, and quenching the steel at a cooling rate that leads to hardening, by means of the austenite to martensite transformation. The microstructural change associated to this transformation imply a volume expansion that leads to a pressure overshot in the pressing tools (320), which is currently not detected in industrial practice, but which can be measured with the aforementioned disposition of the pressure sensor (100).

[0033] The pressure measurements of this phenomenon, captured by the force gauge (110) of the pressure sensor (100), are received at the processor (200) through second connection means (220), complementary to the first connection means (130) of the pressure sensor (100) and analyzed by data processing means (210). Any input data, processed data and/or final output result may be stored in the memory (230) and/or displayed through a user interface (240). The processor (200) may be implemented on a computer, a digital signal processor, a field-programmable gate array, an application-specific integrated circuit, a micro-processor, a micro-controller, or any other form of programmable hardware, either in a separate device from the pressure sensor (100), or integrated in a joint device with the pressure sensor (100). Likewise, the user interface (240) may comprise any number of input and/or output sources, such as buttons, screens, keyboards, microphones, speakers, touch screens, etc.

[0034] Note that the connection between the force gauge (110) and the data processing means (210) may include more than one communication link. Also, in particular embodiments of the invention, the data processing means (210) may be located in a different equipment as the user interface (240), such as a remote server, accessed by the user device through a communication network.

[0035] Furthermore, in particular embodiments of the invention, the results of the data processing means (210), that is, the real-time detection of the finalization of the quenching process may be used to automatically control the stamping means (300). For example, when the data processing means (210) locate an overshot (630) in the measured time-resolved pressure curve (600), a signal may be sent to an actuator connected to the stamping means (300), ordering said stamping means (300) to release the current steel sheet (400). Alternatively, the real-time detection of the finalization of the quenching process may be notified to a user through the user interface (240) for manual operation of the stamping means (300), or displayed for quenching process refinement in later iterations.

[0036] Figure 2 is a sectional view which schematically represents the operation of the first embodiment of the system of the invention. A steel sheet (400) is shaped and quenched by the pressing tools (320) of the hot stamping means (300), namely a lower die of fixed position and a complementary upper die which is pressed in a downward direction. In this case, the pressure sensor (100) is located in the upper die, near an area of the steel sheet (400) previously determined to be the slowest-cooling region (420) of said steel sheet (400), which in this case corresponds to a central area of the steepest surface of the pressing tools (320), that is, the surface of the pressing tools (320) which forms a smallest angle with the pressing direction.

[0037] The shaping and quenching processes of the steel sheet (400) performed by the stamping means (300) comprises four stages:

• Firstly, the austentized steel sheet is heated in a furnace and transferred to the stamping means (300), where radiation and convection processes occur. The steel sheet (400) in austenitized condition presents a lower volume than the same steel sheet (400) in quenched condition, as the density of the martensite is lower than the density of austenite.
• Secondly, the pressing tools (320) of the stamping means (300) approach the steel sheet (400) until first contact is achieved. In this stage, radiation and convection phenomena concur.
• Thirdly, the forming of the steel sheet (400) is carried out in a single stroke of the pressing tools (320), which are displaced from the position of first contact down to the Bottom Dead Center position (BDC). In this stage, conduction and convection phenomena take place.
• Finally, the pressing tools (320) are kept in the stroke's BDC until the steel sheet undergoes the martensitic transformation. That is, the austenite is transformed to martensite, leading to a volume expansion of the steel sheet (400). In this stage, conduction occurs and the steel sheet (400) is quenched. This last takes most of the time, as quenching in all areas of the shaped steel sheet (400) needs to be guaranteed before removal.

[0038] During the final stage, if the steel sheet (400) was free of constraints, the thickness increase would increase the gap between the lower and upper die of the pressing tools (320). However, since the position of said dies of the pressing tools (320) remains fixed, the steel sheet is constrained between the dies and the pressing tools (320) exert a compressing force against the steel sheet (400), maintaining its thickness. As a consequence, an overshot (630) from the stamping pressure value achieved at the end of the third stage (620) in the time-resolved pressure curve (600) can be detected by the processor (200), which send a signal to inform that quenching has been performed, which can be used to immediately open the press and extract the part.

[0039] Bulk and contact thermal conductivities and tool cooling systems are of utmost importance to reduce the duration of the quenching step. For example, high conductivity steels, conformal cooling strategies and studies of the Interfacial Heat Transfer Coefficient (IHTC) between the steel sheet (400) and the pressing tools (320) have been developed. Nevertheless, pressure force is not distributed uniformly on the steel sheet surface during the quenching step, for example due to angle differences between the steel sheet (400) surface and the pressing direction of the stamping means (300). This leads to IHTC differences along the stamped steel sheet (400), that increase the cooling rate variability already caused by tool shape, contact sequence and cooling channel performance. As a result, the martensitic transformation is not performed in unison but sequentially, being those areas with lowest IHTC the latest to transform, i.e. the slowest-cooling regions (420). The total quenching time that the pressing tools (320) must be kept in the BDC position for a total martensitic transformation must thus account for the lowest local pressure sites, as they drive the lowest IHTC values.

[0040] In order to determine the slowest-cooling region (420) of the steel sheet, and hence place the pressure sensor (100) in a position of the stamping means (300) capable of monitoring said slowest-cooling region (420), several alternative strategies may be followed:

• Designing a specific slot in a die of the stamping means (300) placed in a spot with delayed quenching conditions. In order to guarantee said delayed quenching conditions, the surface where the slot is implemented is recommended to be flat, forming a small angle with the pressing direction of the pressing tools (320). An equidistant position to nearby cooling channels of the die, is also recommended, as well as direct contact between the pressure sensor (100) and the steel sheet (400).
• Performing cooling rate numerical simulations by means of any computational technique for heat transfer modelling known in the state of the art.
• Performing hot stamping trials and identifying the latest hardening spot by mapping the surface of the formed steel sheet (400) through infrared thermography, immediately after being released from the stamping means (300), and selecting the hottest point.
• Repeating the manufacturing process for a plurality of increasing quenching times. The minimum, maximum and sampling step of the series of quenching times is chosen so that at least one manufactured sample is properly quenched, whereas at least one manufactured sample shows at least one area with inadequate hardening. If multiple areas present inadequate hardening, the sampling step may be reduced until one sample only comprises a single area with inadequate hardening, which will be designed as the slowest-cooling region (420) for the purpose of pressure sensor (100) placing. Proper hardening can be checked by any means such as metallographic inspection, hardness surveys or tensile testing.

[0041] Figure 3 presents the main elements of a second preferred embodiment of the system of the invention, which implements the steps of a second preferred embodiment of the method of the invention. The second preferred embodiment is configured to measure the force increase due to the austenite to martensite phase-transformation on a removable protrusion (430) attached to the main stamping body (410) of the steel sheet (400). The pressure and temperature conditions applied to the removable protrusion (430) are calibrated to match the quenching dynamics of the slowest-cooling region (420) of the main stamping body (410) of the steel sheet (400) being quenched.

[0042] In this second preferred embodiment, the pressure sensor (100) also comprises a force gauge (110) and attaching means (120), with the difference that the attaching means are configured to attach the pressure sensor (100) to an auxiliary receptacle (520) in an auxiliary structure (500). That is, when the pressing tool (320) of the stamping means (300) are in contact with the main stamping body (410) of the steel sheet, the auxiliary structure (500) comprises auxiliary pressing means (510) which are in contact with the removable protrusion (430). In this disposition, the force gauge (110) of the pressure sensor (100) monitors pressure variations generated by the quenching of said removable protrusion (430).

[0043] Note that, with the added design flexibility provided by this second preferred embodiment, the first connection means (130) and second connection means (220) may either be wired or wireless. Also note, that the terms "auxiliary receptacle" and "auxiliary pressing means" are meant to merely indicate that said elements are part of an auxiliary device partially or fully independent from the stamping means performing operating on the main body of the steel sheet. Therefore, in this context, the term "auxiliary" is used for the sake of clarity, and does not impose any particular restriction on the receptacle and pressing means design.

[0044] The removable protrusion (430) is a lateral extension of the main stamping body (410) which is removed from the final product after the hot stamping process is finished, and which acts as a reference to monitor the quenching dynamics of the slowest-cooling region (420) of the main stamping body (410) of the steel sheet (400). An example of a preferred geometry for the removable protrusion (430) is shown in figure 4.

[0045] During system operation, the removable protrusion (430) is pressed between the pressure sensor (100) and a flat surface by the auxiliary pressing means (510). The cooling rate of said removable protrusion (430) hence depends on the pressing conditions of the auxiliary structure (500), such as pressing force, operation temperature and cooling conditions. In order to emulate quenching behavior of the slowest-cooling region (420) in the removable protrusion (430), i.e. inducing the same cooling rate, the auxiliary structure (500) may comprise calibration means which tune said pressing conditions. That is, the auxiliary structure is configured to guarantee that when the main stamping body (410) of the steel sheet (400) is fully quenched, so is the removable protrusion (430); and when the slowest-cooling region (420) of the main stamping body (410) isn't fully quenched, neither is the removable protrusion (430). This equivalence may be verified by metallographic means, hardness surveys or tensile testing.

[0046] In order to ensure that the stamping of the removable protrusion (430) is synchronous with the stamping of the main stamping body (410), the auxiliary pressing means (510) of the auxiliary structure (500) may have common or connected elements with the pressing tool (320) of the stamping means (300). For example, both the auxiliary pressing means (510) and the pressing tool (320) may share a common flat surface and be driven by synchronized pressing forces. Cooling fluid may also be shared between cooling channels (330) of the stamping means (300) and the auxiliary structure (500). Furthermore, the flow of cooling fluid may be tuned by means of a flow control valve (530), in order to guarantee the equivalence of the cooling rates of the removable protrusion (430) and the slowest-cooling region (420) of the main stamping body (410).

[0047] Alternatively, or in combination with the cooling fluid flow control, the auxiliary structure (500) may comprise mechanical tools to modify the cooling rate of the removable protrusion, such as a hydraulic actuator (540) or a controllable spring whose rigidity can be tuned to modify the pressure conditions applied to the removable protrusion (430). In both cases, the control of the mechanical tools may be fixed during the whole operation, or may include a closed control loop for adaptive configuration.

[0048] Figure 5 presents a schematic example of a monitoring system relying on an auxiliary structure (500) for enabling the pressure sensor (100) to measure pressure variations on the removable protrusion (500). Note that in this case, the auxiliary structure (500) and the stamping means (300) share the lower die and the cooling channels (330), whereas a hydraulic actuator (540) is used to connect the auxiliary structure to the upper die of the press tools (320). This configuration provides synchronization between the hot stamping of the main body (410) of the steel sheet (400) and the removable protrusion (420), while also enabling calibrating the quenching rate of the removable protrusion (420) by controlling the force exerted by the hydraulic actuator (540). Note that in this case, the pressure sensor (100) is not in direct contact with the removable protrusion (500). Instead, a force transmitting element (150), such as a rod or a punch, is disposed between said pressure sensor (100) and removable protrusion (500).

[0049] Note that in alternative embodiments of the invention, a servo-hydraulic or servomechanic device connected to a controller may be used. The controller may be in direct communication with the stamping means, or synchronized with the stroke of said stamping means by means of a mechanical cam, a Linear Variable Differential Transformer, a laser position sensor, or any other synchronization means known in the state of the art.

[0050] Alternatively, a hydraulic cylinder mounted in the upper die of the stamping means may be used. The working force of the hydraulic cylinder is fixed with a limiting valve until a pressure equivalent to the pressure exerted by the stamping means (300) on the slowest-cooling region (420) at BDC position is reached. Then, the hydraulic cylinder is by-passed to an anti-return valve until the quenching process is determined to be finished. The hydraulic cylinder may also be replaced by a mechanical spring mounted on the upper die which comes to a mechanical stop when the quenching conditions of the slowest-cooling region (420) are emulated in the removable protrusion (430).

[0051] Figure 6 presents an example of a time-resolved pressure curve (600) measured by the force gauge (110) of a pressure sensor (100) during the whole hot stamping process of a steel sheet (400). This particular non-limiting example, corresponds to a four strain gauge Wheatstone bridge in contact with a 80x90 mm surface of AlSi-coated 22MnB5 steel, with a thickness of 3 mm.

[0052] The time-resolved pressure curve (600) presents a first ascending ramp (610) that reaches a first pressure threshold (620) when BDC is reached (620). That is, the first threshold (620) is due to the pressure induced by the press tools (320) at BDC position in the absence of volume alterations of the steel sheet (400). Without the austenite to martensite phase transformation occurring during quenching, this first threshold (620) would be the maximum of the time-resolved pressure curve (600). However, this transformation leads to an overshot (630), that is, a pressure increase not caused by the pressing tools (320) but from the volume increase of the steel sheet (400). The presence of the overshot (630), or more preferably, the stabilization of said overshot (630) is detected by the data processing means (210), which determine that quenching is finalized and send a signal that lead to press tool (320) opening. Finally, said press tool (320) opening reflects in the time-resolved pressure curve as a steep pressure drop (640).

[0053] The presence and/or stabilization of this overshot is detected by the data processing means (210) by any general data analysis technique for overshot detection known in the state of the art, such as comparison of the time-resolved pressure curve (600) with a second threshold (higher than the first threshold, and previously determined during a calibration step or my numerical or theoretical analysis), or by detecting a magnitude variation of the derivative function of said time-resolved pressure curve (600). For example, overshot stabilization (i.e. quenching finalization) may be determined when it is verified both that the time-resolved pressure curve (600) exceeds the first pressure threshold (620) by a predefined amount. Alternatively, any other data analysis technique known in the state of the art that enables to differentiate the contribution to the time-resolved pressure curve (600) of the press tool (320), and the contribution of the volume expansion of the steel sheet (400) may be applied.

[0054] Note that the strength and visibility of the overshot (630) depends on the operating conditions (temperature, pressure, etc) of the stamping means (300) and the characteristics of the steel sheet (400) (material, coating, thickness, etc). For example, a small peak caused by a force excess of the control mechanics of the press tool (320) before stabilization at BDC may be detected right after reaching the first threshold (620). The data processing means (210) may therefore include mathematical conditions in its decision algorithm to avoid misclassifying said peak as an overshot (630), such as setting a minimum time duration before a pressure increase is considered an overshot (630).

[0055] In another example, if quenching starts very quickly, the flat area with the value of the first threshold may be reduced or shadowed, with the overshot (630) starting right after the first ascending ramp (610). The data processing means (210) may therefore include mathematical conditions to avoid missing the overshot (630), such as an accurate calibration of the first threshold (620), without imposing the detection of the aforementioned flat area; or by detecting the start of the overshot (630) as a reduction in the derivative function of the time-resolved pressure curve (600).

[0056] In order to facilitate or optimize detection of the overshot (630), the method may comprise a previous calibration step where the time-resolved pressure curve (600) is measured for different operating conditions of the stamping means (300), and the parameters (or range of parameters) where overshot detection is optimized (i.e. maximum overshot visibility) is selected for posterior hot stamping iterations.

[0057] Note that the particular output signals generated by the processor (200) based on the analysis of the time-resolved pressure curve (600) may vary depending on each particular embodiment. Said output signals may comprise, for example, some or all of the following:

• Typically, when the quenching process is considered to be finalized due to overshot (630) stabilization, a first output sign is sent to the stamping means (300) indicating die opening and part extraction; and a second output signal is sent to the furnace and transfer elements in order to feed the next steel sheet (400) to be stamped.
• If quenching finalization is not detected for a given period of time, an alarm signal may be sent in order to indicate the line operators of a possible problem. If quenching finalization is not detected in a single case, it may be caused by a faulty steel sheet (400); whereas quenching finalization in several instances, it may be caused by a faulty batch, for example including an excess of Boron.
• In a more complex configuration, the processor (200) may provide real-time instructions to actuators on hot-pressing parameters (such as press force or cooling flow), based on overshot (630) monitoring, in order to keep a line working at a constant takt time.
• Any other alarm, line to press synchronization signal, or whole line interlock signal.

Claims

1. System of quenching monitoring during steel hot stamping, the system comprising:

- stamping means (300) comprising at least one pressing tool (320) adapted to form and quench a steel sheet (400);

characterized in that the system further comprises:

- at least one pressure sensor (100) configured to measure a time-resolved pressure curve (600) of at least a region of the steel sheet (400); and

- a processor (200) configured to detect an overshot (630) in the measured time-resolved pressure curve (600) generated by volume alterations of the steel sheet (400) during quenching, and to determine when said quenching is finalized based on the detected overshot (630).

2. System according to claim 1 characterized in that the at least one pressure sensor (100) further comprises attaching means (120) adapted to attach said at least one pressure sensor (100) in a complementary sensor receptacle (310) located in the stamping means (300).

3. System according to claim 2 characterized in that the sensor receptacle (310) is located in an area of the stamping means (300) which, during hot stamping of the steel sheet (400), provides pressure measurements of a slowest-cooling region (420) of said steel sheet (400).

4. System according to any of claims 2 and 3 characterized in that the at least one pressure sensor (100) further comprises first connection means (130) configured to establish a wireless communication with second connection means (220) of the processor (200).

5. System according to claim 1 characterized in that the at least one pressure sensor (100) further comprises attaching means (120) adapted to attach said at least one pressure sensor (100) to an auxiliary receptacle (520) of an auxiliary structure (500), being said auxiliary structure (500) adapted to place the pressure sensor (100) in a position where said pressure sensor (100) provides pressure measurements of a removable protrusion (430) of the steel sheet (400) during quenching.

6. System according to claim 5 characterized in that the auxiliary structure (500) further comprises calibration means adapted to emulate quenching properties of the slowest-cooling region (420) of the steel sheet (400) in the removable protrusion (430).

7. System according to claim 6 characterized in that the calibration means of the auxiliary structure (500) comprise a hydraulic actuator (540).

8. System according to any of claims 6 and 7 characterized in that the calibration means of the auxiliary structure (500) comprise a flow control valve (530) configured to control a flow of cooling fluid shared between the auxiliary structure (500) and the cooling channels (330) of the stamping means (300).

9. Method of quenching monitoring during a steel hot stamping process where at least one pressing tool (320) forms and quenches a steel sheet (400), characterized in that the method comprises:

- measuring a time-resolved pressure curve (600) of at least a region of the steel sheet (400);

- detecting an overshot (630) in the measured time-resolved pressure curve (600) generated by volume alterations of the steel sheet (400) during quenching; and

- determining when quenching is finalized by locating an overshot (630) in the measured time-resolved pressure curve (600).

10. Method according to claim 9 characterized in that the step of measuring the time-resolved pressure curve (600) further comprises measuring a force induced by a slowest-cooling region (420) of the steel sheet (400) on at least one pressure sensor (100).

11. Method according to claim 9 characterized in that the step of measuring the time-resolved pressure curve (600) further comprises measuring a force induced by a removable protrusion (430) on at least one pressure sensor (100) located in an area which, during hot stamping of the steel sheet (400), wherein the removable protrusion (430) emulates quenching properties of the slowest-cooling region (420) of the steel sheet (400).

12. Method according to any of claims 9 to 11 characterized in that the method further comprises determining the slowest-cooling region (420) of the steel sheet (400) by means of numerical simulations.

13. Method according to any of claims 9 to 11 characterized in that the method further comprises determining the slowest-cooling region (420) of the steel sheet (400) by means of prior experimental testing.

14. Method according to any of claims 9 to 13 characterized in that the method further comprises determining an optimal range of hot stamping force where overshot (630) detection is optimized, by means of measuring the time-resolved pressure curve (600) generated by volume alterations of the steel sheet (400) during quenching for a plurality of hot stamping forces.

15. A computer program comprising computer program code means adapted to perform the steps of the method according to any of claims 10 to 14 when said program is run on a computer, a digital signal processor, a field-programmable gate array, an application-specific integrated circuit, a micro-processor, a micro-controller, or any other form of programmable hardware.

Drawing