Advanced High Strength Steels Steels Research Articles

Characterizing the mechanical deformation response of AHSS steels: A comparative study of cyclic plasticity models under monotonic and reversal loading

This study evaluates the performance of a user-defined combined hardening modeling method for advanced high-strength steels (AHSS) under monotonic and reversal loading conditions. The plastic behavior of TWIP980 and TRIP980 AHSS sheet metals is investigated using a cyclic plasticity modeling approach. The model incorporates an isotropic von Mises yield criterion and a single-term Chaboche nonlinear kinematic hardening rule. Monotonic and reversal loading stress-strain curves are predicted and compared with experimental results. The model accurately captured the Bauschinger effect for both materials, but it needs help to effectively model the permanent softening behavior observed in TWIP980 steel. Overall, the proposed modeling method agrees well with experimental results for monotonic loading and accurately represents the Bauschinger effect and transient behavior during reversal loading. However, better improvements are needed to capture the permanent softening behavior of TWIP980 steel.

The Influence of Small Additions of Alloying Elements on the Hot Ductility of AHSS Steels: A Critical Review Part 2

In this paper, the influence of small additions of Cr, Mo, Cu, Ni, B, Ca, Zr, and Ce on the hot ductility of advanced high-strength steels (AHSS) has been reviewed. Most of these small additions have a positive effect in improving hot ductility on straightening during continuous casting operations and should be considered when problems with cracking in continuous casting are encountered. In many of these cases, the reason for these generally small but important improvements in hot ductility is not known with certainty, but the segregation of these elements to the austenite grain boundaries, strengthening the bonding, is often suggested.

A study on HAZ behaviour in 800 MPa cold rolled and hot rolled steel weld

Abstract In the automotive sector, the demand for advanced high strength steels (AHSS) is increasing day by day. Based on the application, cold rolled and hot rolled steels are used for various components in a vehicle body. Typically, cold-rolled grades with dual-phase, DP780 steel is used in the form of welded blanks. Hot rolled grades with Ti-Nb microalloy content, like HS800 steel are used in as long members. Welding is an important step to be considered in the design of materials for mass production as required in the automobile sector. In this investigation, Pulsed Gas Metal Arc Welding (P-GMAW) is carried out on both steels with a solid filler wire of strength 800 MPa. Static tensile tests indicated that failure in both steels welds occurred in the heat-affected zone region. The crack initiation and propagation behaviour were compared in both steels. In DP780 steels, the presence of the acicular ferrite and acicular martensite resisted the crack initiation and propagation in the weld region whereas, the chaotic nature of the acicular ferrite in HS800 steel. Microstructural studies revealed that the reason for heat affected zone (HAZ) failure in HS800 steel is due to the presence of hard TiN particles with a size of more than 1 μm which causes decohesion in the matrix. In DP780 steel failure is due to the presence of tempered martensite in the subcritical heat affected zone (SCHAZ). This study divulges the influence of filler wire chemistry, dilution, and welding parameters on cold rolled and hot rolled AHSS steels used in the automotive industry.

Electrochemical Noise Measurements of Advanced High-Strength Steels in Different Solutions

Advanced high-strength steels (AHSS), are commonly used in the manufacture of car bodies, as well as in front and rear rails, and safety posts. These components can be exposed to corrosive environments for instance, in countries where de-icing salts are used. In this work, the corrosion behavior of four AHSS steels with dual-phase [ferrite-martensite (DP) and ferrite-bainite (FB)] steels were studied by means of electrochemical noise (EN) measurements according to the ASTM G199-09 standard in NaCl, CaCl2 and MgCl2 aqueous solutions at room temperature. The direct current (DC) trend data from EN were removed by a polynomial method of statistical and spectral analysis. According to the noise resistance (Rn) values obtained for the DP and FB dual-phase steels, both the martensite/bainite content and morphology of the phase constituents have an important effect on the corrosion behavior of these steels. The L.I. (localization index) (0.00054 to 0.15431), skewness (−6.18 to 7.35) and kurtosis (high values 37.15, 74.84 and 106.52) were calculated. In general, the results indicated that the main corrosion process is related to uniform corrosion. Corrosion behavior of AHSS steels exposed in NaCl solution could be related to the morphology of the phase constituents exposed in NaCl, CaCl2 and MgCl2 solutions.

Numerical study on variation of chord modulus on the springback of high-strength steels

The advanced high-strength steels (AHSS) have become an interesting alternative to the automotive industry to reduce vehicle weight and therefore reduce fuel consumption. However, the wide variety of applications in the automotive industry is still limited due to challenges in its formability and unloaded behavior of these steels popularly called as springback. Computational tools for numerical simulation have been employed in the industrial environment to help predict the occurrence of springback and defining the appropriate parameters to eliminate or reduce their magnitude. However, the accuracy of the numerical results for AHSS still failed to reach a satisfactory level. The limitation in predicting the springback of AHSS by means of finite element method (FEM) is assigned to computationally difficult to characterize the mechanical behavior of these steels during the plastic strain. The variation of elastic modulus during plastic strain is considered as a major cause of non-linearity of the behavior of these steels. This work aims to study the declining behavior of the modulus of elasticity with an increasing plastic strain objecting to an improvement in the computational prediction of the springback phenomenon of AHSS. Specimens of various AHSS steels were loaded and unloaded in uniaxial tension in 0°, 45°, and 90° to the rolling direction. For all AHSS, it was found that the elastic modulus decreases during loading and unloading after each acquired plastic strain approximately up to 10% of plastic strain and then saturates at higher strain values. Further, the L-bending test was simulated where the change of elastic modulus with respect to plastic strain as observed in the uniaxial tension test is informed through the user subroutine VUSDFLD material model. The springback results were then compared with the experiments. It was found that predictions are in close agreement with experiments after informing the model with the decline of elastic modulus with respect to plastic strain through user subroutine material model as compared to the baseline model.

An approach for the design of multiphase advanced high-strength steels based on the behavior of CCT diagrams simulated from the intercritical temperature range

Most of information regarding the construction of continuous cooling transformation (CCT) diagrams considers the decomposition of austenite above the critical transformation temperature Ac3. The development of new grades of multiphase advanced high-strength steels (AHSS) usually involves an intercritical annealing stage, making it necessary the construction of CCT diagrams from the intercritical range. The availability of CCT diagrams constructed from the intercritical austenite is practically non-existent in the open literature. The present research reports an approach for the design of AHSS based on the behavior of CCT diagrams calculated from the intercritical temperature range as a function of chemical composition. Changes in chemical composition were conducted to promote the fabrication of multiphase AHSS steels under conditions that simulate continuous annealing and galvanizing lines (CAGL). To validate the results obtained from computer simulations, the steel was then fabricated, processed at laboratory scale and subjected to thermal cycles that simulate CAGL. Although there is a good approximation between theoretical and experimental results, it was observed that the software presents some limitations regarding the effects of plastic deformation and carbon partitioning during the isothermal bainitic treatment (IBT) on the final microstructures.

USE OF TTT AND CCT PHASE TRANSFORMATION DIAGRAMS FOR MANUFACTURING ROUTE DESIGN OF ADVANCED HIGH STRENGTH STEELS

Advanced High Strength Steels (AHSS) have been developed to meet the demands of the automotive industry, which are focused on making the use of fuel more efficient, with a special interest in reducing the weight of vehicles but without sacrificing the safety of passengers. Microstructural control is the key in the development of these steels, making it possible to obtain different degrees of resistance but with the same chemical composition. In the present work, TTT and CCT diagrams were used as a potential tool for designing an appropriate thermal treatment for the manufacture of two AHSS steels. The CCT and TTT diagrams were simulated using the JMat-Pro? software version 8.0. The critical temperatures of transformation A1 and A3, were determined experimentally by dilatometry. The steel was heat treated following two routes. In the first route a two-phase microstructure consisting on ferrite+martensite was generated, which is typical of dual phase steels (DP). The second was designed to obtain a multiphase microstructure consisting on ferrite+retained austenite+bainite+martensite, which is typical of transformation induced plasticity (TRIP) steels. The strength of the two types of steels was evaluated by uniaxial tensile tests, obtaining average values of ultimate tensile strength and % elongation of 1067 MPa and 7.51 % for DP steel and 890 MPa and 30.75 % for TRIP steel, respectively. These values are in accordance with those expected for this type of steels. The results show that CCT and TTT diagrams can be used to determine optimal processing conditions for the development of AHSS steels, by which time and processing costs can be reduced. KEYWORDS: TRIP steels, simulation, advanced high strength steels, double phase steel, multiphase steel.

Phases quantification in DP600 steel welded by GTAW process using SEM and atomic force microscopy

The automotive industry is constantly under several challenges in many aspects, such as development of new materials and improvement their manufacturability. In order to achieve light weight, reduced emissions and ensure conductor safety, advanced high strength steels (AHSS) are able to fulfill these requirements. Dual phase steels (DP) are well suited for light weighing car body constructions. The gas tungsten arc welding (GTAW) process is focused in literature as an alternative choice for joining AHSS steels; this study is held to disclose the exhibited microstructural constituents. In addition, quantitative determinations of the volume % of phases in the various weld regions were made. The relative amounts of lower bainite (LB), upper bainite (UB), and polygonal ferrite (PF) in the heat affected zone (HAZ) were determined by image analysis from optical microscopy (OM) and scanning electron microscopy (SEM). It was found that GTAW promoted the development of significant amounts of LB in the HAZ (50.89 %). In contrast atomic force microscopy (AFM) leads to quantify the different phases by the morphology, height and roughness, it was found that martensite (M) dropped down to 7.5% in the intercritical zone (IZ) although PF increases to 92.5% compared with the base metal (BM).

Methodology to assess fracture during crash simulation: fracture strain criteria and their calibration

The use of Advanced High Strength Steels (AHSS) has greatly increased this last decade in the automotive industry. Because of their crash performance and their weight saving potential, these grades constitute a possible solution to achieve the safety and environmental regulations objectives. Nevertheless, the increase of tensile strength of these materials is generally associated with a loss of ductility compared to conventional steels. Thus, the prediction of their failure in crash loading conditions become of great importance for the design of vehicles. This paper proposes to calibrate for any AHSS, as Dual-Phase or Press Hardened Steels, different failure criteria, available in finite element software. First, specific tests and methodologies for strain measurement needed for models calibration are exposed. Second, an overview of these tested models and the procedure applied to take into account mesh dependency are provided. Finally, simulations results are compared with experiment on a real automotive component.

Mechanical and Metallographic Effects of Laser Hardening of Two AHSS Steels

Abstract The search for suitable materials for the transport industry, to meet more stringent regulations related to crashworthiness, emissions, and fuel economy led to the development of advanced high-strength steels (AHSS). Thermal treatments, for example, using lasers as a process tool, can locally improve some characteristics of these materials like plastic deformation capability. The high energetic efficiency of a laser process, and its capability to be automated, are some of the advantages of using such a process. This study aims to investigate the effects of local heat treatment (involving changes in the solid state), by laser radiation, in the mechanical properties and microstructures of two types of advanced high-strength steels, the dual-phase DP 600, and the transformation-induced plasticity TRIP 750. A method to evaluate the interaction between laser radiation and the materials is proposed. Previous studies in this area focused, basically, in welding or cutting technology, and for the modern TRIP steel studied here, there is a scarcity of published material regarding laser–material interaction. Hardness and tensile tests revealed, for the range of process parameters studied, an improvement (up to 30 % with relation to the base material) in yield strength and ultimate strength (up to 15 %). Revealed also is a dramatic reduction in the elongation (up to 80 %) for both materials. Optical metallography analysis revealed that the resulting microstructures presented grain refinement and formation of lath martensite according to the level of laser absorption, improving up to twice the original hardness.

Role of the advanced microstructures characterization in modeling of mechanical properties of AHSS steels

Detailed knowledge of the fraction, morphology and chemical composition of phase constituents and their effect on the mechanical properties play a crucial role in understanding of the mechanisms influencing the properties of Advanced High Strength Steels (AHSS). On the other hand, the most important microstructural features of these steels are characterized by different size, starting from the nano- and ending on the microscale. Therefore, a detailed characterization of the AHSS microstructure must involve many methods capable of tracing the microstructure at different scale levels. The paper presents selected capabilities of advanced analytical techniques, in combination with conventional light optical microscopy (LOM), for quantitative characterization of the microstructure developed in AHSS steels during thermomechanical processing or continuous annealing. The material used for the investigation comprised the samples of DP steel sheet produced at the industrial scale. Special emphasis was focused on the capabilities of the Field Emission Gun Scanning Electron Microscopy (FEG SEM) combined with EBSD of microstructural characterization. The significance of accurate microstructure characterization for the modeling of mechanical properties of AHSS steels was demonstrated for the case of numerical calculation of the stress–strain curve in the standard tensile test. The work results indicate that such an engineering approach is useful for prediction of material properties.

Hydrogen Embrittlement of Automotive Advanced High-Strength Steels

Advanced high-strength steels (AHSS) have a better combination between strength and ductility than conventional HSS, and higher crash resistances are obtained in concomitance with weight reduction of car structural components. These steels have been developed in the last few decades, and their use is rapidly increasing. Notwithstanding, some of their important features have to be still understood and studied in order to completely characterize their service behavior. In particular, the high mechanical resistance of AHSS makes hydrogen-related problems a great concern for this steel grade. This article investigates the hydrogen embrittlement (HE) of four AHSS steels. The behavior of one transformation induced plasticity (TRIP), two martensitic with different strength levels, and one hot-stamping steels has been studied using slow strain rate tensile (SSRT) tests on electrochemically hydrogenated notched samples. The embrittlement susceptibility of these AHSS steels has been correlated mainly to their strength level and to their microstructural features. Finally, the hydrogen critical concentrations for HE, established by SSRT tests, have been compared to hydrogen contents absorbed during the painting process of a body in white (BIW) structure, experimentally determined during a real cycle in an industrial plant.

Assessment of Damage Models in Sheet Metal Forming for Industrial Applications

Due to increasing demands to reduce C02-emission and to augment occupant’s safety new modern materials are developed ongoing. Because of relatively low production costs, high strength and simultaneously good formability the advanced high strength steels (AHSS) are applied among others for the lightweight design of body-in-white components in the automotive industry. Their already mentioned properties follow from the presence of mixed mild and hard ferrous phases. Due to this multiphase microstructure of the most AHSS steels, a complex material and damage behavior is observed during forming. The damage grows in a ductile manner during plastic flow and the cracks appear without necking. They are often characterized as the so called shear cracks. The damage predictions with standard methods like the forming limit curve (FLC) lack accuracy and reliability. These methods are based on the measurement of linear strain paths. On the other hand ductile damage models are generally used in the bulk forming and crash analysis. The goal is to prove if these models can be applied for the damage prediction in sheet metal forming and which troubles have to be overcome. This paper demonstrates the capability of the Gurson-Tvergaard-Needleman (GTN) model within commercial codes to treat industrial applications. The GTN damage model describes the existence of voids and they evolution (nucleation, growth and coalescence). After a short introduction of the model the finite element aspects of the simulative damage prediction have been investigated. Finally, the determination of the damage model parameters is discussed for a test part.

Yb:YAG laser welding of TRIP780 steel with dual phase and mild steels for use in tailor welded blanks

Advanced high strength steels (AHSS) are essential to meet the demands of safety and fuel efficiency in vehicles. In this paper, we present the results of laser welding of two AHSS steels, TRIP780 and DP980. A 2 kW Trumpf TRUDISK 6002 ® Yb:YAG laser beam was utilized to join 1 mm thick TRIP780 with 1.5 mm thick DP980 and 1 mm thick mild steel. Optical metallography was used to characterize the weld profile and microstructures. Microhardness, tensile and fatigue tests were performed to evaluate the mechanical properties. Results indicate that the laser welds exhibit excellent strength and hardness with minimal defects which are attributed to the high beam quality, disk type of laser. In addition, there is a distinct effect of pre-straining of TRIP780 steels on the energy absorption.

Advanced high strength steels for automotive industry

The aim of this paper is to present the basic concepts of advanced high strength steels (AHSS) for use in the automobile industry, including chemical composition design, microstructure and mechanical properties development during thermomechanical processing, production technology characterisation, potential applications and performance in service. AHSS steels are considered to be the major materials for future applications in this production sector. As opposed to the cold formable single phase deep-drawable grades, the mechanical properties of AHSS steels are controlled by many factors, including: phase composition and distribution in the overall microstructure, volume fraction, size and morphology of phase constituents, as well as stability of metastable constituents. The main feature of these steels is that they do not permit to rely on the well-established traditional microstructure-properties relationships. Therefore, many different alloy concepts and alternative processing routes are still under development by different steel producers for comparable steel grades.