Experimental investigation of a new low-temperature hot stamping process for boron steels

Schematic illustration of the experimental setup for both the conventional and new hot stamping processes is shown in Fig. 2(a) and (b), respectively. In a conventional process, the equipment consisted of a furnace for heating the blank, a feeder to quickly transfer the blank, and a stamping press with a set of B-Pillar tool assembly. After heating, austenitized boron steel was quickly transferred to the stamping press and placed on the B-Pillar tool for forming the product with martensitic microstructure. In the new process, a pre-cooling stage was incorporated between the furnace and the stamping press.

Both a conventional hot stamping process of boron steel and a new process are described in Fig. 1, using a red line and a green line, respectively. As shown in Fig. 2, for each forming experiment, an electrically heated roller hearth furnace was used for heating the boron steel. To prevent decarburisation and scale formation, the heating was performed in the nitrogen atmosphere. The material was heated up to 750 °C at a targeted heating rate of 7.5 °C/s followed by a slower, targeted heating rate of 1 °C/s up to 900 °C and held at 900 °C for about 60 s for homogenisation, suggested by Tata Steel. Corresponding to a new process, the austenised boron steel was quickly transferred to pre-cooling stage for rapid cooling to a desired start stamping temperature. Pre-cooling was done at a cooling rate of greater than 60 °C/s with the help of compressed air at 10 bar pressure. Pre-cooling plates which consist of multiple tiny holes were adopted to ensure uniform cooling during the pre-cooling process. To monitor and control the temperature of the blank material during transfer and pre-cooling, calibrated non-contact digital infrared thermometers (pyrometers) were attached to both the feeder and stamping press. Due to fast cooling, even though the austenised boron steel was pre-cooled to a low temperature down to below 500 °C, the material could still be in the austenitic condition for hot stamping. Under a designed forming speed, the stamping process was intended to complete before the austenite start to break down into martensitic, which is around 425 °C [28]. Subsequently, in-die quenching for 20 s further decreased the temperature of boron steel to below martensite transformation finish temperature (280 °C) and resulted in an ultra-high-strength product predominately with martensitic steel.

The transfer time from furnace out to positioning on the stamping die at various stages is listed in Table 2, which is specific to this hot stamping setup and can be further reduced by adjusting the distance between the furnace, pre-cooling device, and stamping press. By defining different pre-cooling time, different start stamping temperatures can be resulted. As shown in Table 2, without pre-cooling (i.e. the pre-cooling time is 0 s), the start stamping temperature is 765 °C, which represents a conventional stamping process; by increasing the pre-cooling time, the start stamping temperature could drop gradually, which represents the new stamping processes with deformation taking place at a lower temperature range.

The setup of the B-Pillar die set installed on a hydraulic press is shown in Fig. 3a. The work mechanism of the die set is illustrated through the schematics given in Fig. 3b: (1) Pad from the bottom of the tool pushes the blank against the punch with 200 kN force, simultaneously, blank holder moves very close to the blank with spacer clearance of 5 mm. (2) Punch moves down to form the B-Pillar shape. (3) Finally, at the end of forming during the press hardening process, punch pushes the blank against die and pad, and instantaneously, blank holder closes completely in order to cool the blank quickly to make martensitic component.

In this study, the B-Pillar demonstration components were defined as two drawing depth, 50 mm and 64 mm, by changing the die sets. For the different drawing depths, the same blank shape was used. The geometry and dimensions of the blank profile, determined through preliminary studies, are shown in Fig. 3c, with a picture of a heated blank on roller given in Fig. 3d. For each drawing depth, hot stamping experiments were conducted at a range of different start stamping temperatures (420–765 °C) and at a range of different forming speeds (60–350 mm/s). It is noted that the temperatures are actual start stamping temperatures confirmed through the non-contact digital infrared thermometer installed on the press. For 765 °C, no pre-cooling was applied; for the rest of the temperatures, pre-cooling was applied. Under the same forming speed, it should take longer to form a component with greater drawing depth, thus the blank would experience greater temperature drop during forming due to heat loss to cold dies, which indicates a higher potential of crossing the martensite phase transformation temperature. Therefore, when pre-cooling was applied, the temperature range defined for forming the components with 64 mm depth (listed in Table 4) was slightly higher than that for 50 mm depth (listed in Table 3).

The success of a forming process should be validated by the fact that the as-form components can achieve both the required geometry (e.g. thickness) and required properties. Therefore, two sets of post-form examination were designed and carried out on the hot-stamped B-Pillars.

Hot-stamped B-Pillars were analysed in terms of maximum thinning, measured by using a set of 3D optical blue light scanner. The complete arrangement of the scanner, movable stand, and a B-Pillar marked with reference points is shown in Fig. 4. In order to obtain the thickness distribution, scanning was performed at both inside and outside surfaces of the B-Pillar. The scanned point clouds were transformed into 3D polygon surfaces (refer to Fig. 5).

To confirm the hot stamping property requirements, tensile and micro-hardness samples were prepared from B-Pillar parts; those were hot stamped through both the conventional and new hot stamping processes. The specimen selection and property evaluation are only for relative comparison between the conventional and new hot stamping process. The sample cut out region in B-Pillar is shown in Fig. 5. Hardness and tensile tests were carried out for various stamping conditions. Tensile tests were performed using ASTM E8 sub-size specimen as shown in Fig. 6.

Fig. 7 shows selected results from hot stamping B-Pillars with 50 mm depth, at the speed of 150 mm/s and different temperatures: (a) shows the component formed through the conventional hot stamping process without pre-cooling, at the start stamping temperature of 765 °C; (b) and (c) show the products stamped through the new hot stamping process with pre-cooling to enable the start stamping temperature to be at either 440 °C (refer to Fig. 7b) or 420 °C (refer to Fig. 7c). B-Pillars were successfully formed without any fractures using both the conventional process (refer to Fig. 7a) and the new hot stamping process with start stamping temperature of 440 °C (refer to Fig. 7b). However, when the start stamping temperature decreased to 420 °C through pre-cooling, the product fractured as shown in Fig. 7c. The stamping process was unsuccessful at all forming speeds due to the formation of hard martensite phase at 420 °C which resulted in product fracture. Table 3 lists the summary of forming results under different process conditions to form products with 50 mm depth.

The depth of final product influenced the success rate of the new stamping process. When the product depth was increased from 50 to 64 mm, the B-Pillar fractured at the start stamping temperature of 450 °C (refer to Fig. 8d). This is due to the formation of martensitic phase at below 420 °C [29] resulted by longer duration of deformation.

The forming speed also influenced the success rate of the new stamping process. For B-Pillar with the depth of 64 mm, the product formation was successful in the conventional hot stamping processes at 765 °C for all three forming speeds of 60 mm/s, 150 mm/s, and 350 mm/s (refer to Fig. 8a). The forming was also successful in the new low-temperature hot stamping processes at 500 °C, with the forming speeds of 150 mm/s and 350 mm/s (refer to Fig. 8b). However, fracture occurred when the forming speed was reduced to 60 mm/s (refer to Fig. 8c). This is because, at a lower forming speed (60 mm/s), stamping time was longer which means additional time was available for heat loss from the blank to dies. It led to much reduced ductility due to martensite phase transformation. Table 4 lists the summary of forming results under different process conditions to form products with 64 mm depth. Fig. 9 shows the comparison of force–displacement curves for two stamping conditions, which are 765 °C, 150 mm/s and 500 °C, 150 mm/s, representing the conventional and new low-temperature hot stamping processes respectively. The forming force was increased by approx. 29% for the new process, which was clearly due to the increased flow stress when the material is deformed at a lower temperature. The fact that the decrease of stamping temperature leads to increased stamping force is noted, which would accordingly increase the mechanical work needed for the stamping. However, it is not the most important factor to be considered in hot stamping because the major energy consumption is for heat treatment and the forming force is relatively low compared with cold forming.

The formability in both conventional (without pre-cooling) and new (with pre-cooling) hot stamping processes were further investigated by measuring the thickness distribution of the final products. Fig. 10 shows the thickness distribution of hot-stamped B-Pillars with a depth of 64 mm formed at the speed of 150 mm/s through both a conventional hot stamping process (without pre-cooling) at the start stamping temperature of 765 °C (refer to Fig. 10a) and a new hot stamping process (with pre-cooling) at 500 °C (refer to Fig. 10b). The maximum thinning of products formed at 765 °C and 500 °C was 29% and 25%, respectively. Fig. 11(a) to (d) shows the thickness of the critical section of B-Pillars (refer to Fig. 5 for critical section) formed under different start stamping temperatures: 765 °C without pre-cooling and 700 °C, 590 °C, and 500 °C with pre-cooling. The maximum thinning was similar in the range of 25–29%. Minimum thickness in the nearly flat region around point B was also measured by using a mechanical round-headed micrometre, to compare the measurements from the 3D scanner. Results were summarised in Table 5. It could be seen that the errors were around ± 0.01 mm (less than ± 1%). It could be stated that the new process with pre-cooling, at the start stamping temperature of 500 °C, allowed more uniform thickness to be obtained, compared with the B-Pillar formed at 765 °C. The reason could be that, at a lower stamping temperature, boron steel blank might have better temperature uniformity during forming. In addition, a higher value of strain hardening for the material at a lower deformation temperature could be another factor to restrain localised deformation. The similar observation was found in deep drawing of aluminium alloy 7075 using hot stamping process where the thickness uniformity was better at a lower temperature range [30] (Fig. 11).

The room-temperature tensile properties of workpieces, which were cut from as-formed parts produced through different hot stamping processes and conditions, were tested and representative results are presented in Fig. 12. The results confirmed that a similar strength level (~ 1400 MPa) was achieved through both the conventional and the new hot stamping processes, for both product depths. The strength is the essential property requirement in hot stamping for UHSS automotive parts. Figure 13 shows microstructure and micro-hardness analysis of boron steels hot stamped under representative conditions through both the conventional processes (765 °C, 150 mm/s) and the new processes (700 °C, 150 mm/s; 590 °C, 150 mm/s; 500 °C, 350 mm/s). The initial Vickers hardness of the as-received material was 170 HV. The Vickers hardness values for all the successful parts were confirmed to be greater than 450 HV, which indicates that martensitic transformation was achieved in both the hot stamping processes, which was further proven through microstructure observation by using scanning electron microscopes (SEM).

To understand the significance of the new hot stamping process in reducing cycle time, in-die quenching experiments were carried out. The in-die quenching tests were performed on a 250-kN hydraulic test machine as shown in Fig. 14a and the test setup is shown in Fig. 14b. The entire tool setup was mounted onto two gas spring cylinders and the required die-workpiece contact pressure was controlled by varying the gas pressure. The quenching dies, including top and bottom dies, were made from AISI H13 steel and were heated to required temperatures by using electric band heaters. Two band heaters were placed around both top and bottom die peripheries, which could ensure uniform heating compared with other heating techniques [31]. A circular boron steel blank with the diameter of 80 mm and the thickness of 1.5 mm was used as the test-piece. The cross-sectional schematics in Fig. 14c show the arrangement of dies, band heaters, and a test-piece under testing. Three pairs of K-type thermocouples were rigidly inserted into the bottom die and the thermocouples are 3.19 mm (TC2), 3.83 mm (TC3), and 4.68 mm (TC4) away from the contact surface of the bottom die. TC2 was used to send the control signal to the temperature controller. According to the difference between measured and set temperatures, the temperature controller applies an electrical current to band heaters. Thermocouples TC3 and TC4 were connected to thermocouple data logger to record the temperatures during the quenching period. A pair of K-type stainless steel thermocouple (TC1) in 0.5 mm diameter was embedded into test-piece in parallel to the blank surfaces, not to affect the contact between the workpiece and dies during in-die quenching. During testing, a test-piece was heated to a desired temperature in an electrical furnace; then, it was quickly transferred to a quenching tool and was pressed between pre-heated top and bottom dies. It is noted that the pre-cooling is very difficult to control without employing a fully automatic system; hence, to imply the experiments in laboratory, the material was heated to a temperature just above the start quenching temperature (rather than austenitisation completion temperature) for each test condition. The temperature histories of both workpiece and dies were recorded during the entire process. To investigate the in-die quenching time required to reach 250 °C (which is below martensite transformation finish temperature), a range of combinations of initial tool temperatures, contact pressure, and workpiece start quenching temperature were defined and detailed in the next subsection.

The influence of workpiece start quenching temperature, initial die temperature, and die-workpiece contact pressure on in-die quenching time during hot stamping processes was analysed and is presented in Figs. 15, 16, and 17.

A summary of the in-die quenching experimental results is presented in Fig. 18 to demonstrate the advantage of the new hot stamping processes in quenching time reduction.

In a conventional hot stamping process (defined start quenching temperature of 740 °C), the in-die quenching time required to cool down the product to 250 °C was 6.3 s for the lower (1 MPa) and 4.4 s for the higher (20 MPa) contact pressure. However, in the new hot stamping process with pre-cooling (defined start quenching temperature of 520 °C), in-die quenching time required was 5.7 s and 2.3 s for the lower and higher contact pressure, respectively. Therefore, it is valid to claim that the new process can reduce the in-die quenching time by 48% or above. It is noted that the decrease of the in-die quenching time is nearly the same as the additional pre-cooling time for the new process, which means the total process time for a standalone cycle of the conventional process and the new process would be nearly the same. However, for continuous volume/mass production, since the parts would be shifted from one stage to the other at the frequency of the slower one (which is the forming and in-die quenching stage here), the new low-temperature hot stamping process is beneficial in the way of increased productivity.

It is worth mentioning that, due to the simplification of the thermal cycle, the data obtained from this set of tests would not be identical as the data obtained from real hot stamping processes involving phase transformations. This is because the thermal diffusivity of the different microstructures would be different [34], while the material microstructure in this set of tests was mainly ferrite-pearlite through the whole heating and cooling cycle, without phase transformations of ferrite-pearlite to austenite and austenite to martensite. In addition, the martensite transformation during cooling would cause the release of the latent heat and the increase of volume. The former one would result in increased heat to extract thus potentially longer cooling time; the latter one would potentially result in increased interfacial heat transfer coefficient thus less cooling time. This work did not include detailed studies on this aspect. According to references [34, 35], the effect of latent heat during cooling caused by austenite to martensite transformation would become neglectable when the contact pressure is high (20 MPa or above). In addition, the difference in thermal diffusivity would be within 10%. Therefore, it is valid to claim that the trend founded through this set of experiments is correct and the exact values of cooling time are not accurate but could be used as a reference for industrial applications.

It is also worth mentioning that, for the new process, the thermo-mechanical fatigue cycle of the die material will be different, with larger stresses but lower temperatures, which would potentially have negative and positive impacts, respectively, on the die duration. This may be important for the die life and could be an interesting point for future investigation.