AHSS AUTO STAMPING CHALLENGES: RECTIFYING SPRINGBACK

Advanced High Strength Steels or AHSS (Advanced High Strength Steels) have superior properties to conventional steel are widely used in the automotive industry, they provide weight reduction and at the same time, increase vehicle safety in the event of collisions.Flangeability is an essential element for forming automotive components such as chassis and suspension components.Research is ongoing into the development of forming techniques that can improve the performance of AHSS during industrial operations, particularly flange drawing.Therefore, there is great interest in investigating the extensibility of the pre-cut cutting edge and determining the factors that significantly influence the formability of the product.The test used to evaluate flangeability is the Hole Expansion Ratio (HER).In this research, the objective was to evaluate the influence of the conditions of four edge cutting methods on the hole expansion test of advanced multiphase high-strength steel class 780 MPa through four methods: Machining, LASER and punching with expansion of the hole in the direction of the punch (burr downwards) and in the opposite direction (burr upwards).The results of this study corroborate the literature that indicates that the state of the edge after cutting directly influences the expansion capacity of the hole.

Challenged to improve safety and fuel economy, automakers continually search for new materials to meet high standards. Several factors drive the material R&D and selection for automotive applications, including safety, fuel efficiency, environmentalism, manufacturability, durability, and quality. In the highly competitive automotive industry, cost is an extremely important factor in material selection. As the motivation to reduce the mass of vehicles continues to grow, automakers seek to maximize the efficiency of their materials selection. Materials in automotive applications are selected to minimize weight while meeting key criteria, including crash performance, stiffness, and forming requirements. Since the 1920s, steel has been the material of choice for automakers worldwide. The weight percentage of steel used in vehicles relative to other materials has grown from around 50% in the early 1980s to about 60% in 2010 for North American light vehicles. Today, steel makes up around 65% of an average automobile’s weight and is the backbone of the entire vehicle. On average, that is 900 kg of steel used per vehicle. To further enhance passenger safety, vehicle performance, and fuel efficiency, reducing the weight of vehicles has become one of the top priorities for the automotive industry. Advanced high-strength steels (AHSSs) are a new generation of steel grades that provide much higher strength and other advantageous properties than other materials while maintaining the high formability required for manufacturing. AHSSs help engineers meet requirements for safety, efficiency, emissions, manufacturability, durability, and quality at a low cost. The relevance of AHSSs is quickly increasing in the automotive industry, and AHSSs are the key material for vehicle mass reduction. Different types of AHSS help parts meet the varied performance demands in different areas of the vehicle, including both the crumple zone and passenger compartment.

To contribute to the environmentally friendly concept; trends and directions of vehicle manufacturing in the world automotive industry have focused on energy-saving vehicles and carbon-monoxide emission reduction. These affect Thai manufacturers to produce light and safe automobiles (the next generation) using lightweight materials and advanced high-strength steels (AHSS). The objectives of this study were 1) to study the readiness of enterprises in the Thai automotive-parts industry in producing automotive parts by using lightweight materials and advanced high-strength steels, 2) to gather their limitations in producing lightweight automotive parts, and 3) to seek recommendations from specialists in Thai Automobile industry to deal with the limitations. This study employed a qualitative methodology. The research instrument was an in-depth interview. The participants were 17 administrators of car-assembling companies, executives from Tier 1, Tier 2 and Tier 3 automotive parts companies/suppliers, and specialists from the automotive industry. The data gained were analyzed through content analysis. The results from this study indicated that small-sized companies in the automotive industry still lacked the knowledge and experience to produce their parts using lightweight materials, requiring external specialists to support them. On the other hand, big-sized companies and joint-venture companies are able to produce their parts with lightweight materials by themselves. In addition, the specialists recommended guidelines for the die design and production from the start to the end stage. The lightweight materials would be more widely used in the Thai automotive industry if their costs were approximately the same as the current materials.

The world’s most common alloy, steel, is the material of choice when it comes to making products as diverse as oil rigs to cars and planes to skyscrapers, simply because of its functionality, adaptability, machine-ability and strength. Newly developed grades of Advanced High Strength Steel (AHSS) significantly outperform competing materials for current and future automotive applications. This is a direct result of steel’s performance flexibility, as well as of its many benefits including low cost, weight reduction capability, safety attributes, reduced greenhouse gas emissions and superior recyclability. To improve crash worthiness and fuel economy, the automotive industry is, increasingly, using AHSS. Today, and in the future, automotive manufacturers must reduce the overall weight of their cars. The most cost-efficient way to do this is with AHSS. However, there are several parameters that decide which of the AHSS types to be used; the most important parameters are derived from the geometrical form of the component and the selection of forming and blanking methods. This paper describes the different types of AHSS, highlights their advantages for use in auto metal stampings, and discusses about the new challenges faced by stampers, particularly those serving the automotive industry.

The automotive industry is constantly involved in the development of new projects aimed at reducing weight, fuel consumptions and costs while improving passengers? safety. In order to achieve these increasing demands, Advanced High Strength Steels (AHSS) have been introduced in recent years reducing vehicle structure weights and improving the crashworthiness. With the increase in the bearing capacity of crash-relevant structural components, the sheet metal joining techniques such as adhesive bonding and resistance spot welding (RSW) become critical. In order to develop the vehicle structure in these new projects, full-vehicle crash finite element simulations are usually performed. Simplified beam-like models are currently used in these simulations (with thousands of spot welds) to represent RSW joints response. The maximum bearing force of these models are fitted using large experimental campaigns, considering all the main factors that have the highest influence on the fracture response of a welded joint. The objective of this thesis is twofold: (1) to develop a model that is able to partially replace the extensive experimental campaign in providing parameters for the crash simulation simplified spot weld models, and (2) to gain understanding of spot weld joints failure response in order to improve the current simplified models. To achieve these objectives, a detailed spot weld model for the prediction of spot weld failure in joints in AHSS sheets is presented. The presented model includes a definition of the local material properties as well as the geometry features of a spot weld. In addition, an industrially suited fracture criterion, i.e. robust and without a long-term calibration, is used for the prediction of maximum force. An energetic fracture criterion based on the use of elastic-plastic fracture-mechanics is identified as the better suited for the prediction of spot weld failure and joint bearing capacity. The J-integral is evaluated in the weld notch and this value is compared with a material parameter, the fracture toughness, in order to obtain the joint maximum force. The presented detailed FE spot weld model is validated to joints of two different steel grades of the AHSS family usually present in the current vehicle structure, a hot formed martensitic boron-alloyed steel (22MnB5) and a cold formed dual phase steel (DP 980). The validation is performed comparing the results obtained with the finite element model and the experimental results extracted from an extensive loading test experimental campaign where the main factors that have an influence in the spot weld fracture response are considered. The obtained simulated critical forces of the loading tests present good agreement with the experimental ones in all tested configurations. Finally, based on the presented finite element spot weld model, some recommendations are exposed for extending the model for new combinations and loading conditions. The proposed procedure can be used to reduce the long-term characterization campaigns used to calibrate the joints of a new AHSS grade, where fracture is triggered by stress concentration ahead of a notch. Furthermore, some recommendations for the future structure design are given taking into account the information obtained with the present model.

Stringent carbon emission targets and consumer demands for highly fuel efficient and safe vehicles are driving most of the innovations in the automotive industry. In the present era, the prime technological goal of the automotive industry is to design and manufacture commercially viable lightweight vehicles while maintaining the structural performance of the vehicles at the same time. Development of Advanced High Strength Steels (AHSS) is an important step forward in this context. However, plastic deformation induces damage in AHSS. Damage development during the forming process renders the classical failure prediction techniques ineffective, which poses difficulties in designing the forming process. Therefore damage development in these steels has been studied and incorporated in numerical simulations for accurate failure predictions in forming processes and for determination of the product properties after forming. Damage development is anisotropic by nature and shall be considered anisotropic for accurate failure predictions. In this research, anisotropy in damage has been classified into two categories; Material Induced Anisotropy in Damage (MIAD) and Load Induced Anisotropy in Damage (LIAD). MIAD is related to the anisotropy in distribution and shape of second phase particles or impurities and is governed by void/crack nucleation. LIAD is related to the loading direction of the material. The phenomenon of MIAD was discovered during this research. This is the first research showing the occurrence of MIAD in AHSS. The standard Lemaitre anisotropic damage model was modified to incorporate: lower damage evolution under compression, strain rate dependency in damage and Material Induced Anisotropic Damage (MIAD). Viscoplastic regularization of damage models was revisited. This technique proved to be effective in removing the pathological mesh dependence of local damage models. The damage model parameters for the same grade of DP600 were determined. The Modified Lemaitre’s (ML) anisotropic damage model was validated with experiments.

The automotive industry is using more and more advanced high strength steels (AHSS), aiming at reducing vehicle weight, minimizing emission, increasing fuel economy, and improving vehicle safety. This article focuses on the AHSS products recently developed at Nucor Corporation. With innovative chemistry and process designs, as well as a strictly controlled manufacturing operation carried out during the unique CSP® and down-stream processes, these newly developed steels possess finer, more uniform microstructure, with modified morphology and optimum fraction for each phase. Compared to the most commercially available AHSS, these new materials have higher strength-elongation balance, improved formability and stretch flangeability, enhanced impact toughness and crashworthiness, good coating or surface quality, as well as superior weldability and weld fatigue properties. Moreover, the property consistence of these innovative AHSS has been markedly improved.

Influence of selected alloying variations on liquid metal embrittlement susceptibility of quenched and partitioned steels

In this study, experimental and numerical analyses of Forming Limit Diagram (FLD) for Advanced High Strength (AHS) steel grade 980 were performed. Forming limit curve was first determined by means of the Nakazima stretch-forming test. Then, analytical calculations of the FLD based on the Marciniak-Kuczynski (M-K) model were carried out. Different yield criteria, namely, Hill’48 (r-value and stress-based), Yld89 (r-value and stress-based) and Barlat2000 (Yld2000-2d) were investigated. The strain hardening law according to Swift was applied. To identify parameters of each model, uniaxial tension, balanced bi-axial bulge test and in-plane biaxial tension test were performed. As a result, predicted plastic flow stresses and plastic anisotropies of the AHS steel by various directions were evaluated. In addition, effects of the anisotropic yield functions, strain rate sensitivities, imperfection values and work hardening coefficient on the predicted FLD were studied and discussed. It was found that the FLD based on the Yld2000-2d yield criterion was in better agreement with the experimental curve. Accuracy of the FLD predictions based on the M-K theory, especially in the biaxial state of stress, significantly depended on the applied yield criteria, for which yield stresses and r-values of different loading directions were required.

Uncoated advanced high strength steels (AHSS) are being used for non-exposed automotive applications where high strength is required. Due to higher contact pressures and friction forces between the tooling and sheet steel, increased die wear, die build-up and sheet steel surface galling have been observed. Although various countermeasures have been adopted in production to address these issues, the stamping die tryout process has not been revised accordingly. Proper heat treatment of uncoated tryout dies is required to prevent wear in stamping uncoated, cold-rolled AHSS. Whether die build-up and sheet steel surface galling occur depends on the steel surface condition. The intent of this study is to investigate the effects of steel surface topography on die build-up, sheet steel surface galling and friction in forming uncoated cold-rolled AHSS. DP590 and DP980 with different thickness and different finish rolled textures were tested with the bending under tension (BUT) tester to study these effects. A reciprocal cyclic bend test system (CBTS) of modifying bending under tension test was used to investigate wear and build-up between different surface texture steels in a production condition. Results indicate that the finish roll surface texture is found to be the root cause for the die build-up and steel surface galling that occurs during forming with uncoated dies. The ground-roll surface is the worst in terms of galling and build-up while the Electro-Discharge Textured (EDT) surface is the best. The surface parameters Skewness (Rsk) and Kurtosis (Rku) are found to be able to assess steel galling and die build-up behaviors for uncoated cold rolled AHSS using the uncoated D2 die. A surface with Rsk larger than −0.5 and Rku less than 3 can prevent galling and die build-up in the die tryout process, which is recommended to finish roll the uncoated cold rolled AHSS.

Tensile and Spring-Back Behavior of DP600 Advanced High Strength Steel at Warm Temperatures

Galvanized advanced high strength steels (AHSS) will be the most competitive structural material for automotive applications in the next decade. Oxidation of AHSS during the recrystalization annealing process in a continuous galvanizing line to a large extent influences the quality of zinc coating on the final galvanized steel product. For example, formation of oxides of alloying elements (e.g. Mn, Cr, Si) at the steel surface during annealing prior to galvanizing leads to poor adhesion of a zinc coating. Yet, knowledge on the high temperature oxidation behaviour of AHSS is rather limited. The primary aim of this thesis is to provide fundamental understanding on the kinetics of internal oxidation of AHSS during annealing. The classical Wagner internal oxidation theory for binary alloys was extended to account for multi-component alloys. To this end, a generic coupled thermodynamic-kinetic internal oxidation model based on Fick’s 1st law was developed in order to predict the kinetics of internal oxidation, as well as the concentration depth profiles of internal oxides and solute elements in alloy matrix, considering the finite solubility product of oxide precipitates, the non-ideal behaviour of solid solution and the formation of multiple type of oxide species. The internal oxidation behaviour of Fe-Mn and Fe-Mn-Cr steel alloys were experimentally studied to validate the model. It has been found that for Fe-Mn and Fe-Mn-Cr steel alloys, the effect of non-ideal behaviour of solution on internal oxidation is negligible, and local thermodynamic equilibrium is established within internal oxidation zone. Besides, the kinetics of Wustite formation on pure iron and Mn alloyed steels annealed in CO2 + CO or H2O + H2 gas mixtures as well as the reduction kinetics of the Wustite scale in Ar + H2 gas mixtures were investigated. The growth of Wustite scale on iron and Mn alloyed steels follows the linear rate law. However, adding Mn to iron, even at a relatively low concentration (say 1.7 wt%), dramatically lowers the growth rate of Wustite scale. Nevertheless, reduction kinetics of the Wustite scale on iron and Mn alloyed steels are almost the same. During the reduction process a dense iron layer is formed which separates the remaining Wustite scale from the reduction atmosphere. The rate of Wustite reduction by H2 is controlled by the diffusion of solute oxygen dissolved in the formed iron layer.

Resistance spot welding (RSW) is one of the most widely used welding techniques in modern industries due to its efficiency and speed in production, especially in the automotive industry. With the continuous development of engineering materials, advanced high-strength steels (AHSS) such as DP600, DP800, TRIP, and TWIP have emerged, which combine high strength, excellent ductility, toughness, and malleability, making them ideal for structural applications, load and impact resistance, and weight reduction. This makes them play a significant role in improving weld quality, and they are used in cars and aeroplanes due to their superior mechanical performance. In this study, the effect of welding parameters on tensile strength, micro-hardness, and changes in the mechanical properties of spot-welded joints from these advanced high-strength steels (AHSS) is examined. Three parameters were used: current, welding time, and weld thickness. Generally, the quality of the weld depends on the welding standards. In this review paper, the most important studies presented in this field are referenced, all of which indicate that the main welding parameters affecting these materials in the welding process are welding current, welding time, and thickness, and their impact on tensile strength, hardness, and mechanical properties. Studies show that the quality of welding heavily depends on welding parameters.

Forming limit diagram of Advanced High Strength Steels (AHSS) based on strain-path diagram

Hydrogen Embrittlement in Advanced High Strength Steels and Ultra High Strength Steels: a new investigation approach