Press Hardening Research Articles

Hot Stamped Aluminium for Crash-Resistant Automobile Safety Cage Applications

Hot stamping, also known as press hardening in the context of sheet steel, has steadily gained relevance in the automotive industry, starting off as a specialist application and turning into a staple technique in the production of safety cage products in little more than a decade. However, despite the weight reduction offered by martensitic steels, further improvement could be obtained by substituting these components by high-performance aluminium. In this regard, the very same process of hot stamping could be employed to attain the required combination of shape complexity and mechanical properties at a reasonable cost for mass-market application, if the limitations imposed by cycle time and process window could be overcome. In this work, the feasibility of hot stamping of 6000-series aluminium alloy sheet is studied, first in dilatometry experiments and later in semi-industrial conditions in a pilot facility. A cycle time shortening strategy is employed, and compared to the conventional thermal cycle in terms of implementation and obtained results. In addition to basic characterization, aluminium thus processed is studied in terms of fracture toughness, in order to obtain data relevant to crashworthiness that can be readily compared with alternative materials.

Application of Fourier-related data reduction methods in sheet metal forming

Due to the increasing digitalization, modern solutions are needed to work effectively with rapidly growing amounts of data in the automotive production. This paper shows two potential fields of application for the practical use of data reduction methods. In particular, the combination of Fourier-related transformation methods (e.g. discrete Fourier transform) and surrogate models for the useful 3D visualization of complex interrelationships is shown using the example of press hardening and deep drawing.

A novel medium-Mn steel with superior mechanical properties and marginal oxidization after press hardening

A novel Cr-alloyed medium-Mn steel was designed and manufactured for the hot forming application in automobile. Compared to the typical press hardening steel (PHS) grade of 22MnB5, it possesses more excellent mechanical combination and enormously reduced oxidization after press hardening. The synchrotron-based high-energy X-ray diffraction experiment indicated that the solute C atoms could be partitioned from martensite to austenite during the baking process and then segregated on geometry necessary dislocations that had been generated due to the austenite-to-martensite transformation during the previous cooling. This led to much hardened austenite grains, which could bear more load partitioned and accommodate the greater deformation until they transformed to martensite during deformation than the non-baked austenite grains, thus greatly improving both strength and ductility. Moreover, the thickness of oxidization layer on the invented steel is less than 3 μm after the hot forming at 750−810°C, much thinner than the 100 μm-thick oxidization layer on 22MnB5. Such a tremendously reduced oxidation results from both the lower soaking temperature employed for the hot forming and the formation of dense Cr/Al/Si oxide band at the bottom of the oxidation layer, the latter is because the designed steel contains much higher contents of Cr and Al than 22MnB5.

Thermomechanical Treatment of Martensitic Stainless Steels Sheets and Its Effects on Their Deep Drawability and Resulting Hardness in Press Hardening

The deep drawability of three Martensitic Stainless Steels (MSS) alloys under conventional press hardening thermomechanical process conditions was investigated. The three alloys differ in the content of the main elements C and Cr. Firstly, the metallurgical properties of the alloys were determined, i.e., the phase mass fraction diagrams and the concentration of alloying elements in solid solution at the austenitic temperatures with help of the JMatPro® software version 7.0. Derived from this analysis, specific thermomechanical process parameters were defined to evaluate experimentally and numerically the hot sheet formability of the alloys during the deep drawing process. The hot deep drawability of the MSS alloys was experimentally assessed. The hot deep drawability was evaluated with the resulting maximum drawing depth values. In general, all three alloys developed very good formability at forming temperatures between 700 and 900 °C. However, they are susceptible to chemical composition, austenization temperature, dwell time, and flange gap. The hot formability behavior of the alloys as well as the resulting hardness showed very good concordance with the calculated metallurgical values. Finally, a numerical analysis was conducted using Simufact Forming® 15.0 software. The interval time during hot blank transfer to the tool determines the initial and final forming temperature. The effect of the time interval on the forming temperature was analyzed numerically and validated experimentally. It was also possible to determine the maximum level of plastic strain in the deep drawn cup.

TRIP assisted press hardened steel by the anisothermal bainitic ferrite transformation

Abstract A new steel chemical composition is combined with a new press hardening process, in which die-quenching is interrupted by opening the forming tool to permit slow cooling of the hot formed part through the anisothermal bainitic ferrite transformation. This promotes carbon partitioning to austenite before the forming tool is re-closed and die-quenching is resumed to near-ambient temperature. The final microstructure is predominantly bainitic ferrite with dispersions of martensite and up to 11 % retained austenite. Retained austenite can undergo stress induced transformation to martensite in an automobile crash event. The steel exhibits up to 25 % elongation and 930 MPa tensile strength. In contrast to traditional cold formable Transformation Induced Plasticity assisted steels, where retained austenite is consumed during work hardening of cold forming, here, the desired microstructure is achieved after hot forming meaning the retained austenite is more uniformly distributed within the formed part, which enhances energy absorption. The new steel chemical composition is carefully designed to provide optimal microstructural evolution within the constraints of the new press hardening process, yet relatively lean and manufacturer friendly. The new press hardening process is energy efficient as secondary heating is not required since retarded cooling through the bainitic ferrite transformation is provided by residual heat accumulation of the newly developed titanium alloy forming tool. Development of the new technology is demonstrated by press hardening experiments, tensile testing, microstructural analysis, transversal & axial crush testing of formed parts and numerical simulation of crush testing, including a new modelling technique that more accurately simulates deformation of hot versus cold formed parts. Results show a 22 % increase to energy absorption under axial crushing compared to traditional cold formed Transformation Induced Plasticity assisted steels owing to greater work hardening capacity in formed radii of the part, which are shown to be exposed to the highest stresses during crushing.

Experimental Validation of an Intelligent Hybrid Plain Bearing Active Control

Abstract The replacement of damaged journal bearings in forming machines eventuates in very high personal and material costs as well as long-term downtime. Moreover, new discontinuous forming processes (e.g., press hardening and pendulum operations) have a negative effect on the hydrodynamic pressure built up in the bearing and can lead to failure. Therefore, users and manufacturers of forming presses are looking for ways to improve the safety of this key component. In this paper, the concept of an intelligent hybrid plain bearing (IHPB) that combines hydrodynamic and hydrostatic characteristics, presented by Kurth et al., (2019, “Forming 4.0: Smart Machine Components Applied as a Hybrid Plain Bearing and a Tool Clamping System,” Proc. Manuf., 27, pp. 65–71), is detailed and validated on a test rig. The clearance state of the journal bearing is monitored through eddy current sensors and a controlled hydrostatic support is activated, when this state is evaluated as critical. The results show a strong increase in the load capacity and in the minimal lubrication gap of the IHPB within three series of tested experiments: stationary, nonstationary, and press-typical load cases.

Research on the Re-Deformation Characteristics of Hot Stamping of Boron Steel Parts with Tailored Properties

Traditional hot-stamping products have super-high strength, but their plasticity is usually low and their integrated mechanical properties are not excellent. Functionally graded property structures, a relatively novel configuration with a higher material utilization rate, have increasingly captured the attention of researchers. Hot stamping parts with tailored properties display the characteristics of local high strength and high plasticity, which can make up for the limitations of conventional hot stamping and optimize the crash safety performance of vehicles. This new idea provides a means of personalized control in the hot-stamping process. In this paper, a new strategy of local induction heating and press hardening was used for the hot stamping of boron steel parts with tailored properties, of which the microstructure from the hard zone to the soft zone shows a gradient distribution consisting of a martensite phase, multiphase and initial phase, with the hardness ranging from 550 HV to 180 HV. The re-deformation characteristics of hot stamping parts with tailored properties have been studied through the uniaxial tensile test, in cooperation with digital image correlation (DIC) and electron backscattered diffraction (EBSD) techniques. The experiments show that there are easily observable strain distribution characteristics in the re-deformation of hot stamping parts with tailored properties. In the process of tensile deformation, the initial phase zone takes the role of deformation and energy absorption, with the maximum strain before necking reaching 0.32. The local misorientation of this zone was high, and a large number of low angle grain boundaries were formed, while the proportion of small angle grain boundaries increased from 13.5% to 63.3%, and the average grain size decreased from 8.15 μm to 3.43 μm. Meanwhile, the martensite zone takes on the role of anti-collision protection, with a maximum strain of only 0.006, and its local misorientation is mostly unchanged. The re-deformation experimental results show that the hot stamping of boron steel parts with tailored properties meets the functional requirements of a hard zone for anti-collision and a soft zone for energy absorption, suitable for automobile safety parts.

Conductive Heating during Press Hardening by Hot Metal Gas Forming for Curved Complex Part Geometries

Climate targets set by the EU, including the reduction of CO2, are leading to the increased use of lightweight materials for mass production such as press hardening steels. Besides sheet metal forming for high-strength components, tubular or profile forming (Hot Metal Gas Forming—HMGF) allows for designs that are more complex in combination with a lower weight. This paper particularly examines the application of conductive heating of the component for the combined press hardening process. The previous Finite-Element-Method (FEM)-supported design of an industry-oriented, curved component geometry allows the development of forming tools and process peripherals with a high degree of reliability. This work comprises a description regarding the functionality of the tools and the heating strategy for the curved component as well as the measurement technology used to investigate the heat distribution in the component during the conduction process. Subsequently, forming tests are carried out, material characterization is performed by hardness measurements in relevant areas of the component, and the FEM simulation is validated by comparing the resulting sheet thickness distribution to the experimental one.

Influence of Al–Si coating on microstructure and mechanical properties of fiber laser welded and then press-hardened 22MnB5 steel

The de-coated/coated 22MnB5 steel was laser welded and then press hardened in the present study, and the effect of Al–Si coating on microstructure and mechanical properties was comparatively investigated. The results showed that the fusion zone (FZ) of the de-coated joint was approximately composed of 90% (area fraction, the same as below) martensite and 10% (ferrite + bainite), while that of the coated joint contained about 40% martensite and 60% ferrite. The Al–Si coating dissolving in the FZ facilitated the formation of ferrite during the press hardening process. This mixed structure caused a low hardness of 300 HV in the FZ of the coated joint, and other regions composed of regular martensite showed hardness around 500 HV. The tensile strength of the de-coated joint was 1310 MPa, representing 89% joint efficiency relative to the base metal (BM). Ductile and brittle fracture characteristics were both observed. While only brittle fracture occurred for the coated joint showing poorer tensile properties. The digital image correlation (DIC) analysis revealed that the cracks initiated and finally cracked in the FZ for both joints. Both the break bending load and Erichsen value of the de-coated joint were also higher than those of the coated one. The martensite formed in the FZ of de-coated joint was uniform and contained high density of geometrically necessary dislocations (GNDs), which could lead to high strength and ductility due to the coordinated deformation among martensite grains. The large amount of ferrite in the FZ of coated joint led to larger grain misorientation and higher stress concentration.

One‐Step Process for Press Hardened Steel–Carbon Fiber Reinforced Thermoset Polymer Hybrid Parts

A new one‐step process for manufacturing press hardened steel–carbon fiber reinforced (thermoset) polymer hybrid parts with potential for reduced cycle time, infrastructure requirements, and energy consumption compared with traditional two‐ and three‐step processes is developed. The process combines and optimizes the press hardening and prepreg compression molding technologies, traditionally used in isolation for manufacturing press hardened steel and carbon fiber reinforced polymer parts, respectively, to produce hybrid parts in a one‐step, fully integrated process. Heat required for curing and bonding prepreg to steel is provided by residual heat of the steel part immediately following hot forming and interrupted die‐quenching of steel. Thermal conductivity of tool material is investigated to achieve the optimal balance between die‐quenching rate for martensite formation in steel and temperature maintenance for complete curing and bonding of prepreg. Addition of epoxy adhesive and thickness ratio between steel and prepreg are also investigated. Benchmarking is conducted against parts manufactured by the traditional two‐step process, in which the press hardened steel part is formed in isolation before joining with the carbon fiber reinforced polymer part. No sacrifice of part quality is found from the new one‐step process with no loss of mechanical performance, despite clear economic and environmental advantages.

Alloy Optimization for Reducing Delayed Fracture Sensitivity of 2000 MPa Press Hardening Steel

Press hardening steel (PHS) is widely applied in current automotive body design. The trend of using PHS grades with strengths above 1500 MPa raises concerns about sensitivity to hydrogen embrittlement. This study investigates the hydrogen delayed fracture sensitivity of steel alloy 32MnB5 with a 2000 MPa tensile strength and that of several alloy variants involving molybdenum and niobium. It is shown that the delayed cracking resistance can be largely enhanced by using a combination of these alloying elements. The observed improvement appears to mainly originate from the obstruction of hydrogen-induced damage incubation mechanisms by the solutes as well as the precipitates of these alloying elements.

Electrochemical behaviour of 22MnB5 steel coated with hot-dip Al-Si before and after hot-stamping process investigated by means of scanning Kelvin probe microscopy

Press-hardened steels are commonly protected with hot-dip Al-Si coating. Due to the electrochemical complexity of this system, either before or after hot-stamping process, SKPFM was used to investigate the influence of the thermo-mechanical process on the electrochemical behaviour of the galvanic coupling. The hot-stamping process changed significantly the anodic/cathodic coupling of the coating/steel due to iron enrichment in the coating layer. Hence, a concurrent mechanism was thoroughly established, i.e., while the press hardening enhances the corrosion properties of the steel system (steel and metallic coating) through diffusion, at the same time it diminishes the cathodic protection of the Al-Si layer.

Effect of Galvannealed Coating Evolution during Press Hardening on RSW Weldability

Press-hardening steels (PHSs) are used in modern passenger vehicles to increase part strength while reducing vehicle weight to meet both environmental and safety regulations. To prevent oxidization and decarburization during heat treatment, some PHSs are coated with Zn (galvanized or galvannealed). Heating during the press-hardening process drives interdiffusion of Zn from the coating and the Fe from the steel substrate, forming a diffusion layer composed of -Fe phase (a Zn-Fe solid solution). The electrical resistance of the diffusion layer is a function of its thickness and Zn-Fe composition. Both the diffusion layer thickness and the Zn-Fe composition are dependent on the initial coating thickness and heat-treatment time/temperature conditions. Changes to the heat-treatment process shift the resistance spot welding process window by altering the resistance behavior of the material. If the shift in the process window is not accounted for during assembly welding, the welds produced may either be too small or exhibit expulsion, both of which will reduce the strength of the weld. This study showed increasing heat-treatment time shifted the process window toward lower current. A final combined processing window of 1.5 kA, which is suitable for industrial application, was obtained when taking into account the variation in heat-treatment time. The tensile shear performance was not affected by the heat treatment, as increased softening in the heat-affected zone at longer heat-treatment time canceled the strength gain from increasing nugget size.

Microstructure Characterization of Reversed Transformation in Cryogenically Rolled 22MnB5.

Hot stamping is a well-known process to produce structural automotive parts with an excellent strength-to-weight ratio. However, this process is more expensive due to the lower energy efficiency and operating cost of the traditional roller-hearth furnace. Additionally, lower ductility and toughness are commonly recognized as the main disadvantages of the current hot stamped ultra-high-strength parts. Refinement of austenite grains could be a profitable way to improve the strength of hot stamped parts. In this work, the evolution of reversed transformation in asymmetrically cryogenically rolled samples was studied in order to control the austenite. Thermomechanical simulation and heat treatment in the salt bath were used to investigate the reversed transformation process, and the typical microstructures were characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Compared with symmetric prerolling, ferrite recrystallization could be remarkably inhibited by asymmetric rolling at the liquid nitrogen temperature (LNT) during the reheating process. Additionally, the nucleation of the austenite inner grains can also be promoted and the dynamics of the reversed transformation accelerated by asymmetric prerolling. Such phenomena might be very useful to refine the parent austenite grains before press hardening and enhance the new hot stamping strategy by partial fast reheating.

Extension of the Conventional Press Hardening Process by Local Material Influence to Improve Joining Ability

Press hardened structural components are a key factor in lightweight car design and thus in reducing vehicle mass while increasing crash safety. The use of quenched 22MnB5 (Usibor 1500) has been established in hot sheet forming for the production of safety-relevant car body components. In order to expand the field of application for press-hardened components, a process-reliable joining technique is essential. Ultra-high-strength components can be joined with other parts in car bodies using the resistance spot welding process. Here, challenges like uneven welding lens formations with an incorrect connection in multi-sheet joints arise. Mechanical joining processes, for example self-pierce riveting, can only be used to a limited extent due to the high hardness of the hardened parts. For this purpose, an annealing treatment is often carried out in order to reduce the strength of the material after press hardening. Another possibility to create softened areas is the introduction of local deformation in the austenitic material. The phase areas in the continuous cooling transformation diagram are shifted to shorter cooling times, which enables the development of deformation-induced ferrite. The local thinning and softening improves joinability by means of mechanical and thermal joining processes but increases the forming force. Therefore, in this study the process of hot forming and local deformation is first pre-estimated using numerical simulation. The required force for the deformation and an optimal positioning, as well as the possible number of deformation punches, are investigated. Furthermore, the first experimental results of the feasibility of locally thinned and softened sheets are presented. In addition, joining tests by resistance spot welding and self-pierce riveting are carried out on the generated specimen to illustrate the practical effectiveness of the local thinning and the use of deformation-induced ferrite for critical joints.

Investigation of material properties of tailored press hardening parts using numerical and physical simulation

In order to assure good resistance performances, automobile manufacturers are looking for high strength steel, especially for structural parts of body in white, which are formed by hot stamping process. With the aim of guaranteeing specific crash behavior, various technologies have been developed to provide tailored properties.Typical tailored component is the B-pillar, which is studied in this work. In general, the pillar must be with greater resistance in some areas, while in others it must have greater toughness to absorb the energy of a possible impact. The tailored technology investigated in this work is the tailored tempering, in which different areas of the component experience different cooling histories leading to requested final mechanical properties.The methodology used to investigate custom properties involves a first designing phase implemented with Finite Element (FE) commercial programs; FE simulations were performed to investigate the Press Hardening process of a 22MnB5 boron steel blank and in particular the thermo-mechanical cycles related with both the high-resistance and high-toughness regions of the pillar.In the second phase of the proposed approach, the obtained thermo-mechanical cycles have been physically simulated, by using Gleeble 3185 system. The following consecutive steps was reproduced on home-designed specimens: (i) Blank heating for the complete austenitization (at a temperature of 930° C for 4 min). (ii) The heat loss due to the transport phase of the blank from the oven to the press. iii) The mechanical deformation of the blank due to the stamping phase. iv) The quenching phase of the part. (v) The cooling on air of the B-Pillar.Finally, micro-hardness tests have been performed on the specimens subjected to physical simulation. FE model predictions and micro-hardness tests are in good agreement, showing that tailored tempering effectively leads to differentiation of the mechanical properties.

Cracking Behavior of Coating during Hot Tensile Tests of AlSi-Coated Press Hardening Steel

For industrial hot stamping applications, press hardening steel is usually coated with Al-10wt.%Si, in order to prevent substrate decarburization and oxidation at elevated temperatures. However, during hot stamping, the AlSi coating layer fractures, causing severe tool wear, substrate oxidation and increased friction coefficient between the tool and stamped part. The initiation of coating fracture can largely be attributed to the formation of several intermetallic compounds (i.e., FeAl & Fe2Al5) via Fe-diffusion, which also results in void formation throughout the coating layer. These intermetallics are formed mainly during the heating stage, with decreasing Fe-content from the coating-substrate interface. Due to distinctive thermo-mechanical properties of intermetallics compared to the steel substrate, the interaction between different intermetallics, including voids, causes high strain localization around the voids, leading to coating fracture. The goal of this study is to detect the initiation of cracks in a ~45 μm coating layer during uniaxial tensile deformation of a 1.5 mm AlSi-coated press hardening steel. For this purpose, isothermal tensile tests were performed at elevated temperatures. The coating cracks were detected by means of acoustic emission (AE) sensors during deformation. The distribution of coating cracks at hot stamping condition was examined via optical measurements. The tensile strain was measured from a strain grid on the sample gauge. The experiment involves heating the coated steel in a furnace to 920°C for 6 minutes, followed by uniaxial tensile deformation (at 600°C and 800°C), and finally quenching at ambient air. The first AE signal from the sample was observed during the tensile deformation at 600°C, indicating that tensile strain initiates fracture in the coating layer. At cooling stage, the temperature change with time triggered more AE signals, which may correspond to substrate phase transformation and additional fractures in the coated steel; the latter is owing to thermal expansion mismatch between the intermetallics in the coating layer, and steel substrate. Interestingly, no AE signals were observed during the heating stage; i.e., no coating cracks occur prior to deformation and quenching.

Quenching and flash-partitioning enables austenite stabilization during press-hardening processing

Quenching and flash-partitioning (Q&FP) concept is proposed based upon carbon diffusion associated with the martensitic transformation. For the Si-added steel having an elevated martensite start (Ms) temperature, carbon can diffuse rapidly from martensite laths into their adjacent austenite within only a few seconds during the martensitic transformation, to replace carbides precipitation by auto-tempering in the Si-free steel. Elevating Ms in alloy design based on the Q&FP concept enables the stabilization of retained austenite during press hardening. The transformation-induced plasticity effect of retained austenite films improved the total elongation of press-hardened steel to 10.5% combining with 1660 MPa tensile strength.

Predictive Analysis from numerical and experimental data in press hardening

Machine learning, big data and deep learning are today’s catchphrases for how to improve reliability and productivity of your manufacturing equipment. Production companies implement a large number of sensors to record every activity within their production lines and learn as much as possible about their running processes in order to predict shifting product properties and to prevent stoppage due to failure. The successful application of machine learning algorithms to predict machine and process behavior depends on a reliable and balanced database. Since the foremost goal of every manufacturing business is to make sound parts and to avoid defects, there is a large amount of data available for smoothly running processes but only very little for failure production. One approach to solve this imbalance would be to link the production line data with simulation data. Simulation models allow for computing failure parts with no additional costs and therefore enable the exploration of the entire parameter space. We conducted press-hardening experiments with a variation of process parameters for a structural car body part on the press hardening line at Fraunhofer IWU. As an evaluation criterion, we measured the hardness of the final part at critical spots. In order to expand the experimental data, we applied FE simulations to the entire press hardening process chain. The paper explains limitations of the model and elaborates on its parameterization. As a final task, we applied a basic machine-learning algorithm to both experimental and numerical data as well as to their combination in order to evaluate the data space expansion through simulations. The results obtained through machine learning indicate significant differences in the prediction of part quality for solely experimental data and its combination with simulation data. This is especially true for press hardening because of non-linear system behavior and a large amount of uncertain and hard-to-identify parameters. We found that the most challenging parts of uniting measured and simulated data is not only to create simulations with appropriate accuracy, which allow for a meaningful extrapolation of the parameter space, but also to compare simulation and production data based on the same criterion and to have stable simulation models for the entire parameter range.

Parameter Measurement and Conductive Heating During Press Hardening by Hot Metal Gas Forming

Press hardening of steels for the production of car body components is very common. The reason for this is that the process allows the use of blanks with low wall thickness and the production of ultra-high strength components with complex geometries. The Hot Metal Gas Forming (HMGF) process for closed profiles combines the advantages of hydroforming, such as increased rigidity, functional integration or elimination of joining operations, with those of press hardening. In this paper results of continuous tests with actively cooled tools are presented in combination with temperature and displacement measurement. Furthermore, test results for HMGF with tool integrated conductive heating are demonstrated. Tests were done with tube material PHS1800 by SSAB, part temperatures over the Point of austenitisation and maximum internal pressure of 70 MPa. Thermocouples recorded the heat distribution in the tools. Other measured and recorded variables were the displacement of the component wall while forming under increasing internal pressure by a tactile displacement sensor and simultaneous temperature of its surface with a thermal sensor head. For the first time, information on pressure, the corresponding deformation stage and temperature profile could be documented during an entire forming step. A close to series production geometry DP4 was used to investigate the tool-integrated conductive heating of components.