Abstract
The effects of local heating temperature and bending parameters on the formability and springback in the V-bending of galvanized DP600 sheet material are investigated experimentally. In the study, the experiment parameters are determined as temperature (Room Temperature-RT, 100, 200, 300, 400 and 500 °C), die angle (15, 30, 45, 60 and 75) and holding time (0, 10 and 20 sec.). It was observed that there were radical changes in springback angles and formability due to the martensite changes in the microstructure of the material between the temperatures of RT – 500 °C. It was noted that the springback angle increased by 27.46% on average when the heating temperature increased from RT up to 200 °C, decreased by 16.28% on average when the heating temperature increased from 200 °C up to 400 °C, and increased by 22.72 % on average when the heating temperature increased from 400 °C up to 500 °C.
Introduction
The use of advanced high strength steel (AHSS) sheets in the automotive industry has been becoming gradually more prevalent due to its high strength and light weight. The high strength increases the safety of the passenger during an impact, and the light weight decreases the fuel consumption and carbon emissions [1–3]. Dual phase steels, a type of non-quenchable AHSS steel, are low-carbon steels that include ferrite and martensite and they have a high work hardening rate and high formability combination [4–6]. DP steels are predominantly comprised of soft ferrite matrix that has martensitic particle islands. While the ductility of the sheet material is increased due to the soft ferrite, its strength is increased due to the hard martensite [7,8]. One of the most important and common problems that are encountered when forming steel metal materials through bending is the springback behaviour observed in the sheet material after forming. The springback behaviour that is due to the uneven stress distribution among the thick section of the formed part causes shape and measurement defects in the parts [9]. The springback behaviour is related to many parameters such as material characteristics including sheet material thickness, yield strength, elastic module, work hardening, strain rate sensitivity; and process parameters including die angle, holding time, and tool geometry and measurement [10–14]. Solving the springback with trial-and-error methods is a time consuming and costly process. Consequently, knowing the springback values beforehand provides an opportunity to take it into consideration during the die designing process and thus preventing time and material wasting by eliminating the trial-and-error processes.
When compared to ductile materials, high strength materials always exhibit greater springback behaviour [14]. During the unloading process of forming the AHSS sheet material through bending, the measurement sensitivity of the product is much disrupted, and thus greater and unpredictable springback behaviour is observed [3]. It is generally known that forming steels in warm and elevated temperatures is an effective way to eliminate springback. In the literature, there have been studies aimed at reducing and eliminating the springback by heating the material locally or as a whole through various methods. In their study, Öztürk et al. [15] investigated the springback behaviour and the uniaxial tensile strain of advanced high strength DP600 in temperatures ranging from RT to 300°C and in different die directions. They noted that the springback increased until the temperature hit 200°C, and decreased after that point. They also observed that formability was less in 300°C than it was in RT, and that the material exhibited complex behaviours in different temperatures. In their study, Lee et al. [5], suggested a forming process using near-infrared rays (NIRs) to reduce the springback of DP980 dual phase steel which is a non-quenchable AHSS. In their studies where both NIR local heating and heating by an oven are tested separately, they determined that local heating is more advantageous when compared to heating by an oven in terms of both shape sensitivity and hardness. In V-bending, it was observed that oven heating caused springforward, while with NIR local heating methods, the target shape matched with the shaped material in temperatures above 870 K. Löbbe et al. [16] developed a warm bending technology for progressive dies through inline induction heating in order to increase the forming limit and compensate the springback for high strength steels. In their study, an inline induction coil was used to allow a more ductile bending, whereby the absolute temperature was adjusted to compensate springback after unloading. With the developed discrete controller, a stable and fast control mode is achieved. In this way, an initial deviation from the target springback value is reduced. Yanagimoto and Oyomada [17] investigated the non-springback bending mechanism of the high tensile steel sheets under the condition of isothermally heated warm bending. It was observed in the isothermal v-bending tests where induction heating is used that the bending momentum and thus the springback decreased in the steel sheets that are bended in temperatures higher than 750 K. Mori et al. [18], in their study, aimed to improve the springback behaviour and formability of the ultra-high strength steel sheets using resistance heating. The sheet material that was placed on the die was heated directly through electrifying which also called Joule heat. It was observed that during electrifying, the formability and the springback of the ultra-high tensile strength steel sheets that were heated by electrical resistance improved. Moreover, they proved that rapid resistance heating is an effective method to prevent the oxidation and the decrease in the temperature of the sheet material before the bending. Neugebauer et al. [19], in their study, applied local laser heat treatment to the sheet materials in order to improve the formability in deep drawing for high strength steels (DP600, DP1000, MS-W 1200). It was observed that this process increased the stretching and decreased the yield and tensile strength substantially. They have determined that laser heat treatment has the potential to improve the formability of the ultra-high strength steels.
While the warm and hot applications on the sheet material increase formability, it also causes change in the microstructure and decrease in the mechanical characteristics of the whole material. These processes render the sheet material unable to exhibit the expected performance in the fields where it is used. Moreover; while the first setup cost is very high for heating by induction or laser, in the processes where heating is conducted by an oven, as the die and the punch or the material as a whole are heated, the time and energy consumption increases. In V-bending processes where the forming is done throughout a straight line; since just the bending area where the punch touches the sheet material is heated lineally, a small part of the material where it touches the heating rod is affected by heat, the remaining parts of the material are not affected as much. This ensures that the microstructures and the mechanical characteristics of the remaining parts are left unchanged. It was determined that there is not enough research on how the springback behaviour and the formability are affected; and how microstructure affects the springback when local heating is used only on the bending area of the galvanized DP600 sheet material. In addition, such information is needed in finite element simulations. Consequently; as part of this study, the objective is to analyze the effects of the local heating temperature on the microstructure of the material, and therefore on the springback behaviour and the formability of the galvanized DP600 steels during the V-bending process conducted by only heating the bending area between temperatures ranging from RT to 500°C. Moreover, alongside temperature, the effects of process parameters such as die angle and holding time on the springback were investigated.
Material and method
Material
In the experimental studies, galvanized DP600 sheet material (ArcelorMittal DP600 + Z, EN10346:2015-HCT590X + Z) with 1 mm thickness is used. The samples were prepared for the experimental studies by cutting them on rolling direction by guillotine shears 40 × 20 mm in size. The mechanical characteristics of the experiment material were determined by tensile and strength measurement tests, and its chemical characteristics were determined by chemical spectrometer. The tensile tests were conducted through using experiment samples that were cut on a wire erosion bench in accordance with the ASTM-E8 standards and prepared in rolling, diagonal and transverse directions. The images of the experiment samples before and after the test are given in Figure 1. The average values and standard deviations of the mechanical characteristics that are obtained are given in Table 1. The tensile test specimens (a) before and (b) after test.
Mechanical properties of galvanized DP600 sheet metal material.
The preparing of test samples for microstructure investigation
In the preparation process for the microstructure analysis; the experiment samples that were formed in v geometry after RT, 100, 200, 300, 400 and 500°C heating processes were prepared for the bakaliting process by being cut on a sensitive cutting device. After the bakaliting process, the experiment samples were shined by sanding and by felt. For the optical microscope screening process, the experiment samples were kept in 2% mL HNO3 (Nitric Acid) + 98% mL Ethyl Alcohol for 10 s, after were rinsed in water, and thus the etching was completed. After cleaning the solution with water and air, the microstructure images of the experiment materials were obtained and ferrite–martensite phase changes were examined. For every temperature, the microstructure images and the hardness measurements that were obtained were taken from 0.2 mm deep of the interior bending area of the sheet material. The volume rates of the phases inside the DP600 sheet material were measured by colouring the images obtained by Lecia brand optical microscope and by proportioning them according to the surface/area rate. This process was conducted by using the LAS v4.6 software in the microscope.
Experimental setup and parameters
The experimental studies were conducted in an experimental setup where the forming speed, force, heating temperature, and holding time can be monitored electronically. Temperature measurements were done using a thermal camera. The photos of the experimental setup and the experimental study environment are given in Figure 2. The experimental study environment and experimental setup.
The experimental study parameters.
The forming temperature, which is an experiment parameter in the first stage of the experimental study, is adjusted by a proportional–integral–derivative (PID) controlled temperature control panel with ±1°C temperature sensitivity located on the experimental setup. After adjusting the temperature from the panel, the sheet material is placed on the die, and the heating rode is placed on the sheet material lineally where the punch would be in contact with it when it is applied for bending. The information gathered from the thermocouple on the heating rod that obtains the instant temperature changes can be observed from the temperature control panel in real time. Also, the FLIR brand thermal camera controls whether the target temperature is reached. The experiment device is electronically controlled, and the experiment parameters that include forming speed, forming force and holding time can be manually entered as digital data into the control software of the experiment device. In the experimental studies, when the forming temperature on the sheet material reaches the temperature value that is read on the heating rod, the heating rod is removed from the sheet material, and the bending process is conducted. Since the sheet material is thin, it takes maximum of 20 s for the sheet material to reach the forming temperature. Before the bending, the bending punch is held 10 mm above the material-die group. The bending is initiated after the heating rod is removed. The temperatures on the sheet material that are measured by the thermal camera are given in Figure 3. In the thermal camera images that are given in Figure 3, it can be observed that the area that was locally heated was in the forming temperature while the other parts are not heated and remain in RT. The temperatures in experimental study (a) RT, (b) 100, (c) 200, (d) 300, (e) 400 and (f) 500°C.
In the experimental studies, Equation (1) [20] was used to calculate the bending force and was entered in the software of the experiment control device. In this equation; L is press load (N), / is length of bend (mm), t is sheet metal thickness (mm), k is a die-opening factor, S is tensile strength of the sheet metal (N/mm2) and s is the width of the die opening (mm).
Springback behaviour and measurement of springback
Springback is defined as a forming defect originating from the elastic material behaviour which causes geometry and measurement errors in sheet metal forming, especially in bending operations [10]. Within the scope of the experimental studies, hexagon brand coordinate measurement machine (CMM) was used to measure the springback angle in the experiment samples that were formed. First, all of the parts that were formed were pasted on a sheet plate sensitively without any deformation. During the measurement process, Plane 1 was obtained by making the CMM probe contact the formed part from three different points. After this, Plane 2 was obtained by making the CMM probe touch three different points on the second bending area. The springback angle was calculated by measuring the angle between the two planes and extracting it from the bending angle. The measurement process is given in Figure 4. The measurement of springback (a) schematic and (b) CMM.
Results and discussions
The obtained experiment results were evaluated in terms of the effects of the die angle, holding time and local heating temperature on the springback behaviour. In the study, the aim was to prepare the sheet material for bending by only locally heating the area that would be bended, and to ensure that the mechanical characteristics of the remaining parts that were not exposed to heat remained unchanged. Warm forming of sheet material is frequently used to reduce or eliminate the springback in DP sheet materials [21]. In this study, it was also observed that the local heating temperature affected the springback behaviour and the formability of the galvanized DP600 sheet material. The effects of different temperatures on the sheet material were analyzed in terms of ferrite–martensite phase rate change and hardness measurements on the area that was exposed to heat. Microstructure images in different temperatures are given in Figure 5. In Figure 5, white grains represent soft ferrite, while the darker areas contain hard martensite. The microstructure images in different temperatures (a) RT, (b) 100, (c) 200, (d) 300 (e) 400 and (f) 500°C.
In the experimental studies, during the temperature rises from RT to 200°C, it was observed that the martensite phase rate increased from 26.16 to 30.59%. It was determined that the material hardness in the area that was exposed to heat increased from 212 to 230 HV. The increase in the martensite phase rate within the microstructure of the material also increases the hardness of the material. This increase in the hardness of the material causes an increase in the springback angle. Öztürk et al. [15], in their study, observed that the temperature rises from RT to 200°C in the samples that were prepared in rolling direction caused an increase in springback angle. It was also observed in the angle measurements of the experimental study samples that there was an average 27.46% increase in springback angle throughout the temperature rise from RT to 200°C.
During the temperature rises from 200 to 400°C, it was observed that martensite phase rate decreased from 30.59 to 24.94%, and due to this decrease, the material hardness decreased from 230 to 212 HV. The reason for the decrease in the martensite phase was that the martensite started dissolving after 200°C. This condition was observed in ferrite–martensite phase analysis. Similar results were also obtained by Öztürk and et al. [15] in the literature. During the temperature rise from 200 to 400°C, it was observed that there was an average 16.28% decrease in the springback angle.
When the local heating temperature was increased from 400 to 500°C, it was observed that the martensite rate increased from 24.94 to %25.14, and that the material hardness notably increased from 212 to 247 HV. It was detected that due to these increases in martensite phase and hardness, the springback angle increased by an average 22.72%. The ferrite–martensite rates and hardness values according to the phase analysis that was done during the microstructure examination of the locally heated areas of the experiment samples that were heated in different temperatures are given in Figure 6. The ferrite–martenize and hardness variations in local heating area.
Die angle and holding time are important process parameters that affect the springback in bending operations. The springback deformation can be defined as the change in the shape of a part on the removal of tooling due to the unbalanced stress developed over the stamping part during the forming process [22]. It is well known that the springback is due to uneven stress distribution along the cross-section of the component [9]. During the bending operation, compressive stress occurs on the inner surface, and tensile stress occurs on the outer surface of the bending area where the punch touches the sheet material. Because of unbalanced stress distribution across the sheet thickness, the specimen will springback elastically until the internal bending moments are balanced [23]. When the bending angle increases, the bending moment and therefore the stresses occurring during the process increase. Consequently, it was observed that because of the increase in the die angle, there was an increase in the springback angle. In this study, it was determined that the springback angle increased due to the increase in die angle. It was detected that there was an average 26.01% increase in the springback angle of the galvanized DP600 sheet material due to the 15° increase in die angle. The changes in springback angles for different bending die angles in temperatures from RT to 500°C are given in Figure 7. The relationship between springback-temperature variations in different parameters (a) 15, (b) 30, (c) 45, (d) 60 and (e) 75 die angle.
In the bending operation, the period during which the punch is held on the die is called the holding time. Throughout the holding time, the shape of the experiment sample is limited between the die and the punch. This process causes the internal stress relaxation stage which is also called hot-sizing stage. The internal stresses of the specimen decreased during holding because of stress relaxation. This decrease in the internal stress of the experiment sample causes a decrease in the elastic strain and an increase in the permanent strain. It was observed that elastic recovery decreases when holding time increases [24,25]. In the experimental studies, it was determined that there was a decrease in the springback angle of the DP600 sheet material due to the increase in the holding time. However, this increase was not as distinct as the ones that were observed during the changes in temperature and die angle. It was calculated that a 10 s increase in the holding time caused an average 11.87% decrease in the springback angle.
Conclusions
The springback behaviour in the V-bending process that is conducted after locally heating just the bending area of the galvanized DP600 sheet material lineally is investigated experimentally. The main results that were obtained are given below. During the analysis of the experiment samples; it was observed from the thermal cameras that the heat did not affect the parts other than the area where local heating was applied and that the temperature remained in RT in these parts. In the microstructure analysis, it was detected that microstructure and hardness changes occurred in the parts that were affected by the heat. This confirmed that the sheet material could be used without losing its mechanical characteristics, as its parts other than the bending area did not lose their mechanical characteristics. It was observed that local heating in different temperatures between RT-500°C caused significant changes in the microstructure of the bending area that was exposed to the heat, and that this affected formability and springback. In DP600 materials, it was observed that during temperature increase up to 200°C, the material hardness and martensite rate increased and therefore springback increased by an average 27.46% compared to RT. In DP600 sheet material, it was observed that hardness started to decrease since martensite started dissolving after 200°C, and that this continued until the temperature reached 400°C. It was detected that during the temperature increase from 200 to 400°C, the springback angle decreased 16.28% on average. It was observed that during the increase in local heating temperature from 400 to 500°C, the martensite quantity and hardness started to increase again, and this situation caused an average 22.72% increase in the springback angle. Due to the increase in the bending angle, it was observed that the springback increased because of the elastic tensions in the bending area. Moreover, it was also observed that due to the increase in holding time, the tensions were limited, and therefore the springback angle decreased. While a 15° increase in the die angle resulted in a 26.01% increase, a 10 s increase in the holding time resulted in an 11.87% increase in the springback angle on average. In local heating, since the material as a whole or the die are not heated, and also since the process does not require high energy consumption and expensive heating equipment, it is economic. Moreover, it was detected that there were no aesthetical defects on the area which is exposed to heat such as colour change.
