Press Hardening Research Articles

Offline Reinforcement Learning for Adaptive Control in Manufacturing Processes: A Press Hardening Case Study

Abstract This paper explores the application of offline reinforcement learning in batch manufacturing, with a specific focus on press hardening processes. Offline reinforcement learning presents a viable alternative to traditional control and reinforcement learning methods, which often rely on impractical real-world interactions or complex simulations and iterative adjustments to bridge the gap between simulated and real-world environments. We demonstrate how offline reinforcement learning can improve control policies by leveraging existing data, thereby streamlining the training pipeline and reducing reliance on high-fidelity simulators. Our study evaluates the impact of varying data exploration rates by creating five datasets with exploration rates ranging from ε=0 to ε=0.8. Using the conservative Q-learning algorithm, we train and assess policies against both a dynamic baseline and a static industry-standard policy. The results indicate that while offline reinforcement learning effectively refines behavior policies and enhances supervised learning methods, its effectiveness is heavily dependent on the quality and exploratory nature of the initial behavior policy.

Effect of Micro-Crack Depth on Fatigue Property in Zn-Ni Coated Press Hardened Steel

Effect of Micro-Crack Depth on Fatigue Property in Zn-Ni Coated Press Hardened Steel

Effect of Cr on high-temperature oxidation resistance of Cr–Si–Mn alloyed press-hardened steel during press hardening

It is of great interest to overcome the problem of high-temperature oxidation of press-hardened steels (PHS) in press hardening with a cost-effective Cr alloying strategy. Therefore, since Cr is one of the expensive metals, the effect of Cr (Cr ≤ 3 wt%) content on the oxidation resistance of Cr–Si–Mn alloyed steels was investigated. It was found that Cr–Si–Mn alloyed PHS with a Cr content of about 1.5 wt% or less cannot form a continuous Cr2O3 layer during press hardening. However, when the Cr content reaches around 2.5 wt%, the PHS shows excellent oxidation resistance due to the formation of continuous Cr2O3 and amorphous SiO2 layers, as well as an Mn-rich oxide layer. However, further addition of 0.5 wt% could not significantly improve the oxidation resistance of this PHS. Moreover, the increase in Cr content simultaneously enhances the positive role of Si and Mn in high-temperature oxidation resistance. This work provides new insights into the design of the next generation of PHSs with economical and superior performance.

Numerical study on local contact conditions on rough surface under press hardening

Numerical study on local contact conditions on rough surface under press hardening

Development of a digital material shadow for the press hardening route of medium manganese steel

Press-hardened ultra-high strength steel parts are widely used in the automotive sector for their lightweight and safety advantages. Medium-Manganese Steels (MMnS) are being explored as an alternative to boron-manganese steels due to their high strength and ductility after quenching, achieved at lower annealing temperatures thus reducing energy usage and carbon emissions. However, industrial adoption of MMnS is hindered by challenging processing requirements, e.g. in cold-rolling and press hardening.To expedite and improve the process development, data-driven decisions based on process parameters hold promise. Establishing a link between process data and the final produced part necessitates the development of a framework for a Digital Material Shadow (DMS).This paper investigates the development of a DMS framework for the cold rolling and press hardening process chain. In conjunction with conventional data acquisition methods employed for cold rolling, novel data acquisition techniques are introduced specifically tailored for press hardening, ensuring the comprehensive availability of relevant data. Moreover, a data pipeline is implemented to enable automatic processing, visualization, and analysis of process data. To facilitate seamless data linkage across processes in the DMS, an ID-system is introduced. Finally, the developed framework’s validity is demonstrated by creating a DMS for press-hardened MMnS parts, showcasing its potential for practical applications.

Comparative Study of Anti-Corrosion Properties of Different Types of Press-Hardened Steels.

Hot stamping (or press hardening) is a new technology that is widely used in the production of advanced high-strength steel parts for automotive applications. Electrochemical measurements, including potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), and accelerated corrosion tests (the neutral salt spray test and periodic immersion test) were conducted on press-hardened samples produced from uncoated (cold-rolled and cold strip production (CSP) hot-rolled) and Al-Si-coated press-hardened steels to elucidate their distinct anti-corrosion mechanisms. The cross-sectional micromorphology and element distribution of three types of press-hardened steels after a neutral salt spray test were observed using scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDAX). The corrosion resistance of Al-Si-coated press-hardened steel was found to be significantly diminished following the hot stamping process due to the presence of microcracks and elevated iron content in the coating subsequent to austenitizing heat treatment. On the other hand, the corrosion resistance of uncoated press-hardened sheets produced from cold-rolled and CSP hot-rolled press-hardened steel was found to be proximal due to their nearly identical composition and microstructure (fully martensite) after the hot stamping process. Considering the high efficiency and energy-saving properties of hot-rolled press-hardened steel, it holds the potential to replace cold-rolled and even aluminum-silicon-coated press-hardened steel in automobile manufacturing.

Advanced process characterization and machine learning-based correlations between interdiffusion layer and expulsion in spot welding

Over the past decades, substantial endeavors have been dedicated to unraveling the intricacies inherent to Resistance Spot Welding (RSW). However, a comprehensive and consensual understanding of the RSW process physics is still lacking, including the exact number of physical phases behind the RSW process. For example, a widely accepted model indicates that metal only starts melting after the peak of dynamic resistance, while the latest research on welding uncoated materials challenges this by suggesting that melting begins around the resistance peak. Furthermore, most of existing physical models only consider welding materials without coatings in a controlled lab setting, whereas coated sheet metal is the norm in real production. Addressing these challenges, this paper introduces an enhanced model for RSW that considers the melting phase of the coating's InterDiffusion Layer (IDL) in Press Hardening Steels (PHS). This phase is believed to influence both welding quality and the occurrence of expulsions. Additionally, the timing at which each phase starts has been determined by analyzing real-time, multi-variable sensing data from various welding scenarios, and a signal processing technique has been devised to automatically identify when these phases begin. Leveraging this refined process understanding and characterization, meaningful explainable features are extracted, and a data-driven multilayer perceptron model is constructed for 1) predicting IDL thickness and 2) detecting expulsions upon predicted IDL thickness. The experimental results validate that the proposed IDL-inclusive model advances existing physical models for RSW and the IDL prediction improves the RSW defect detection and process monitoring.

Hot Stamping of Heavy-Gaged - Full Development and the Effect of Press Hardening Process on the Microstructure and Mechanical Properties of 22MnB5 + NbMo Alloyed

For saving mass, to improve the payloads and the durability of commercial vehicles, it is necessary to increase the thickness range of PHS - Press Hardening Steel, replacing low strength steel grades on structural and saving parts. This work presents the industrial development of Heavy-Gaged PHS-Press hardening Steel (4-8mm) starting from the alloy design concept, especially adding niobium to promote a fine and homogeneous microstructure. The development included hot forming simulations, in order to achieve martensitic microstructure through the thickness, correlating with cooling rates and mechanical characteristics, after quenching, as well as welding qualification and fatigue evaluation. Furthermore, were presented the characterization of 22MnB5(Nb+Mo) steel after industrial hot stamping that were evaluated mechanical properties, using the Vickers 10Kgf and the relationship between the microstructure and the thermodynamic aspects.

Improving Weldability of Press Hardened Steel Through Combining Stepped Current Pulse and Magnetically Assisted Resistance Spot Welding Process

Abstract Press-hardened steel (PHS), characterized by its extremely high strength, has wide applications in vehicle body manufacturing as an innovative lightweight material. Nevertheless, the poor weldability of PHS results in poor weld toughness and a high risk of interfacial fracture (IF), posing challenges to the resistance spot welding (RSW) process. Introducing an external magnetic field in the welding process to perform electromagnetic stirring (EMS), the magnetically assisted RSW (MA-RSW) process has been proven an effective method to improve the weld toughness of high-strength steel, but it may increase the risk of expulsion. In response to these challenges, this study introduces a new process called SPMA-RSW to improve the weldability of PHS by combining MA-RSW and the stepped-current pulses (SP) technique, which can enlarge the weld lobe. Nugget appearance, microstructure, microhardness, and mechanical properties were systematically investigated by comparing traditional RSW, MA-RSW, SP-RSW, and SPMA-RSW. The result showed that the SPMA-RSW process would significantly increase the nugget size, inhibit the shrinkage voids, finer the grain size of PHS welds, and harden the nugget region. This increased the lap-shear strength and changed the fracture mode from brittle IF mode to ductile plug fracture (PF) mode at the same heat input. Specifically, the peak load and energy absorption were increased by 32.3% and 84.2%, respectively. Then, an analytical model was developed to reveal the mechanism of the effect of EMS on the fracture mode transition and was verified by experiment. This work can help improve the weld quality and thermal efficiency of the RSW process for PHS.

Oxidation mechanisms of a Cr[sbnd]Si alloyed coating-free press-hardened steel under simulated press hardening conditions

Press-hardened steel (PHS) undergoes severe high-temperature oxidation after press hardening, which significantly degrades surface quality and increases manufacturing cost. This work reports the mechanism by which a coating-free PHS (CF-PHS) with the simultaneous presence of Cr, Si, and Mn alloying elements achieves excellent high-temperature oxidation resistance. It is found that Si and Mn in the CF-PHS promote the formation of a continuous Cr2O3 layer with good protective effect, thereby improving high-temperature oxidation resistance, during the press hardening. Moreover, the continuous amorphous SiO2 layer formed at the oxide scale/matrix interface and the Mn-rich oxides that improve the compactness of the scale further enhance the oxidation resistance of the CF-PHS.

Effects of the Intermediate Precooling Parameters on the Mechanical Properties of a Tailored Press‐Hardened Component in Advanced High‐Strength Steel

In recent years, several automotive companies have studied new tailored tempering technologies for press hardening process to produce structural components with tailored mechanical properties, that is, component with both soft zones to absorb energy and hard zones to avoid intrusion during an impact. One of these technologies involves a tailored tempering station interposed between the austenitization furnace and the press machine in which classical stamping and quenching phases occur. In the tailored tempering station, areas that are desired to be ductile are cooled, while areas with the final high mechanical properties remain at the complete austenitization temperature. This work investigates this new tailored tempering technology by means of finite‐element models and experimental lab‐scale tests. The aim is understanding the influence of cooling rate and the dwell time in the tempering station (precooling time) on the formability and mechanical and microstructural properties of the stamped part. Two different cooling ways of soft areas are explored: 1) by a cooled mask and 2) by a mask with compressed air. Results indicate that an increase in precooling time leads to greater softening in the ductile region but also leads to formability challenges. These effects are even more pronounced with a higher cooling rate.

Effect of Micro-Crack Depth on Fatigue Property in Zn–Ni Coated Press Hardened Steel

Effect of Micro-Crack Depth on Fatigue Property in Zn–Ni Coated Press Hardened Steel

Ecological and Economic Feasibility of Inductive Heating for Sustainable Press Hardening Processes

Ecological and Economic Feasibility of Inductive Heating for Sustainable Press Hardening Processes

A Novel 1000 MPa Grade Ultrafine-Grained Dual-Phase Press Hardening Steel with Superior Oxidation Resistance and High Ductility.

1000 MPa grade low-carbon martensite press hardening steels (PHS) are widely used in energy-absorbing domains of automotive parts, such as the bottom of a B-pillar. To prevent oxide scale formation during hot forming, this PHS is often required to be protected by an additional Al-Si coating. In addition, although the low carbon martensitic microstructure grants it excellent bending toughness, the ductility tends to be limited. In this study, a novel 1000 MPa grade ultrafine-grained (UFG) martensite-ferrite (F-M) dual-phase (DP) PHS with superior oxidation resistance was designed using tailored additions of Cr, Mn, and Si, and refining the initial microstructure. Only 0.55 ± 0.18 μm thick oxide film is formed in the designed steel during austenitizing heating and stamping, which is significantly lower than the 24.6 ± 3.1 μm thick oxide film formed in conventional 1000 MPa grade low-carbon martensite PHS under the identical condition. The superior oxidation resistance of designed steel can be attributed to the rapid formation of the protective Si-rich, Cr-rich, and Mn-rich oxide layers during annealing. Moreover, due to the presence of ferrite and ultrafine microstructure, the designed steel also shows a significant improvement in ductility from 8.5% to 16.8% without sacrificing strength and bending toughness compared with conventional 1000 MPa grade low-carbon martensite PHS.

Effect of tool temperature and dwell time in the tool for press hardening on the mechanical properties of modern high-strength steel with 3% Mn

The production of sheet safety parts for car bodies is currently carried out using press hardening technology. This technology enables the creation of complex-shaped profiles even from high-strength steels without a significant spring-back effect. In combination with modern high-strength steels, which have retained austenite in their structure, it is possible to achieve high ultimate strength and ductility values. As a result, these parts are used in bodywork areas with high energy absorption requirements during impact. To achieve the required mechanical properties, suitable processing parameters must be selected. High-strength steel with 0.2%C, 3%Mn and 2%Al was used for the experiment. Press-hardening was carried out in a tool fixed in a hydraulic press and can be heated up to temperatures around 450 °C. Different tool temperatures of room temperature and 425 °C were tested and different holding times in the tool from 1 s to 600 s. After hardening in a tool at RT, the ultimate strength of about 1400 MPa with a ductility of 18 % was obtained. But quenching and holding in the preheated tool caused the ductility to increase to 28% with a drop in ultimate strength to 1050 MPa.

Soft Sensors for Property-Controlled Multi-Stage Press Hardening of 22MnB5

In multi-stage press hardening, the product properties are determined by the thermo-mechanical history during the sequence of heat treatment and forming steps. To measure these properties and finally to control them by feedback, two soft sensors are developed in this work. The press hardening of 22MnB5 sheet material in a progressive die, where the material is first rapidly austenitized, then pre-cooled, stretch-formed, and finally die bent, serves as the framework for the development of these sensors. To provide feedback on the temporal and spatial temperature distribution, a soft sensor based on a model derived from the Dynamic mode decomposition (DMD) is presented. The model is extended to a parametric DMD and combined with a Kalman filter to estimate the temperature (-distribution) as a function of all process-relevant control variables. The soft sensor can estimate the temperature distribution based on local thermocouple measurements with an error of less than 10 °C during the process-relevant time steps. For the online prediction of the final microstructure, an artificial neural network (ANN)-based microstructure soft sensor is developed. As part of this, a transferable framework for deriving input parameters for the ANN based on the process route in multi-stage press hardening is presented, along with a method for developing a training database using a 1-element model implemented with LS-Dyna and utilizing the material model Mat248 (PHS_BMW). The developed ANN-based microstructure soft sensor can predict the final microstructure for specific regions of the formed and hardened sheet in a time span of far less than 1 s with a maximum deviation of a phase fraction of 1.8 % to a reference simulation.

Experimental and Numerical Process Design for Press Partitioning of the New Q&P Steel 37SiB6

Quenching and partitioning (Q&P) heat treatments of low-alloy steels with exceptional property combinations are particularly promising. In this study, we characterize for the first time a new low-alloy steel to be processed using Q&P heat treatments. In combined experimental and numerical studies, we design a novel approach that effectively combines the short cycle times of press hardening with the excellent property profiles of Q&P-treated steels. We identify an appropriate austenization temperature of 950 °C and a portioning temperature of 250 °C for Q&P heat treatments through dilatometric studies. We adjust a number of reference conditions with fractions of 2.1 to 6.3 wt.% of retained austenite, resulting in tensile strengths up to 1860 MPa and elongations to failure up to 7%. Initial numerical designs of the process can identify varying temperature profiles and cooling rates depending on the position in the die. The results show that the geometry of the part plays a minor role, but the die temperature of 200 °C is the dominant factor for successful partitioning directly in the press hardening process.

Crashworthiness evaluation of press hardened steels with different lath-like microstructures.

The deployment of Press Hardened Steels (PHSs) in the Body-In-White structures that took place during the two last decades is at the origin of a breakthrough weight saving on vehicles while guaranteeing the safety requirements of the automotive industry. The production of parts with complex shapes and a high strength between 1500-2000MPa was made possible by taking advantage of the hot rheology of austenite combined with the fast cooling obtained by in-die quenching leading to auto-tempered martensitic microstructures. In the present work, PHS1500 was hot stamped with different thermomechanical processes to promote different lath-like microstructures (bainite, tempered martensite…). An innovative approach is proposed to link the complex microstructures formed between thermoregulated dies to the mechanical behaviour and local ductility of these materials during crash-like solicitations. The microstructure has been characterised through a multi-characterization technique approach including light optical microscopy, scanning electron microscopy and X-ray diffraction. The local ductility is assessed with a combination of crack initiation tests (bending, notch tensile test) and crack propagation resistance tests (Essential work of fracture). Despite similar microstructural crystallographic features, lower bainite and auto-tempered martensite can lead to very different strength – local ductility compromise.

Wear Mechanisms in Press Hardening: An Analysis through Comparison of Tribological Tests and Industrial Tools

Press hardened components have become widespread in the automotive industry in structural and crash-resistant applications, thanks to the combination of the complex shapes and high mechanical properties obtained. However, the press hardening of coated boron steel results in severe adhesive-based wear, with tool maintenance being required in as few as 3000 cycles. The current industrial implementation of press hardening is defined to work around this phenomenon. While this aspect has been studied by different authors, most of the literature deals with laboratory-scale tribosimulators, leaving an open question into how this knowledge transfers to macroscopic effects on the industrial process. In this work, wear in press hardening is studied by comparing the results obtained in laboratory conditions with a pilot-scale line, and finally, with wear mechanisms observed on industrial tools. The aim of this study is to consolidate the current knowledge about the micro-mechanisms involved, and to understand to what extent the existing tests reproduce the actual mechanisms observed in the press floor. The results show how material transfer mainly happens as an accumulation of dust compacted into initial defects on the tool surface. Moreover, this mechanism is effectively reproduced in laboratory tribosimulators and pilot environments, showing a similar morphology to wear on industrial tools. The work sheds light on the underlying causes of wear, and its potential mitigation strategies.

Characterization of Nanosized Carbide Precipitates in Multiple Microalloyed Press Hardening Steels

Press hardening steel standardly relies on titanium microalloying for protecting boron from being tied up by residual nitrogen. This practice safeguards the hardenability effect of boron during die quenching. More recently, additional microalloying elements were added to press hardening steel to further improve properties and service performance. Niobium was found to induce microstructural refinement, leading to better toughness, bendability, and hydrogen embrittlement resistance. In that respect, niobium also extends the operating window of the press hardening process. Vanadium microalloying has been proposed to provide hydrogen trapping by its carbide precipitates. A recently developed press hardening steel employs all three microalloying elements in an attempt to further enhance performance. The current study analyses the microstructure of such multiple microalloyed press hardening steel, and compares it to the standard grade. Particularly, the effect of various heat treatments is investigated, indicating that the multiple microalloyed steel is more resistant against grain coarsening. TEM analysis is used to identify the various particle species formed in the steels, to track their formation, and to determine their size distributions. Nanosized microalloy carbide particles typically comprise a mixed composition involving niobium, titanium, and vanadium. Furthermore, these precipitates are incoherent to the matrix. Regarding tensile properties, it is found that the multiple microalloyed press hardening steel is superior to the standard grade.