Advanced High Strength Steel Grades Research Articles

Modelling of room temperature outgassing and diffusion in a martensitic advanced high-strength steel

Hydrogen embrittlement (HE) is among the limiting factors for the employment of advanced high-strength steels in the automotive industry. One of the most relevant manifestations of HE for those components is delayed fracture. To perform HE risk assessment against delayed fracture in structural components wherein hydrogen accumulates near notches or bends after manufacturing or assembly, continuum models can be used to predict hydrogen diffusion and accumulation. To this aim, it is crucial to identify both the correct model parameters and boundary conditions (BCs). This study provides the required modelling and experimental framework to estimate the constitutive diffusion, trapping and outgassing parameters for a commercial martensitic advanced high-strength steel grade 1300. Permeation testing and simple room outgassing tests showed that a natural BC with one-parameter effectively reproduces the experimental results. The proposed model can be used to predict a reasonable hydrogen distribution, both inside of the component and near to the surface, which is valuable for HE risk assessments. Finally, the same BC can be employed with the interpretation of ex-situ hydrogen charged tensile tests.

Online Grain Size Measurement by Laser Ultrasonics in a Hot Rolling Mill

Laser ultrasonics (LUS) has shown through numerous studies and in-situ laboratory experiments, its suitability to track microstructural features evolution as well as metallurgical transformation in steels during thermal cycles with temperatures reaching 1250°C. This prior work has motivated the use of this technology to be in applied in an industrial hot strip mill (HSM) in Eisenhüttenstadt, Germany. For this purpose, a transportable trolley which embedded LUS components (Generation and Detection Lasers, Two-Wave Mixing Interferometer, Dedicated Fibered Optical Head) was designed. The industrial investigation was carried on moving strips during normal production at several locations of the HSM, resulting in signals for high-strength lowalloy (HSLA) and advanced high-strength steel (AHSS) grades. The development of a specific data processing tool allowed calculating the grain size from the analysis of the online recorded signals. Such results were then compared to predictions from metallurgical models, finding reasonably close values. LUS appears as a suitable solution to monitor microstructural features and metallurgical transformations occurring in steel in a Hot Strip Mill (HSM) line. This approach could enable in the future the development of more reliable models and control loop of the process. This action was supported by the European commission and led by a consortium composed by ArcelorMittal, Imagine Optic and IMI-NRC.

Numerical modelling of shear cutting using particle methods

The use of Advanced High Strength Steel (AHSS) allows for lightweighting of sheet steel components, with maintained structural integrity of the part. However, AHSS grades show limitations in edge crack resistance, primarily influenced by sheared edge damage introduced by the shear cutting process. Numerical modelling of the shear cutting process can aid the understanding of the sheared edge damage, thus avoiding unforeseen edge cracking in the subsequent cold forming. However, the extreme deformations of the blank during the shear cutting process are likely to cause numerical instabilities and divergence using conventional Finite Element modelling. To overcome these challenges, this work presents the use of a particle-based numerical modelling method called the Particle Finite Element Method (PFEM). PFEM accurately solves some of the challenges encountered in shear cutting with the standard Finite Element method, such as large deformation, angular distortions, generation of new boundaries and presents an efficient way of transfer historical information from the old to the new mesh, minimising the results diffusion. The present work shows prediction of cut edge morphology of AHSS using a PFEM modelling scheme, where the numerical results are verified against experiments. With these results, the authors show new possibilities to obtain accurate numerical prediction of the shear cutting process, which promotes further advances in prediction of edge damaged related to shear cutting of AHSS.

Characterization and Modeling of Anisotropic Fracture of Advanced High-Strength Steel Sheets

<div class="section abstract"><div class="htmlview paragraph">As an engineering approach of balanced complexity and accuracy, the Generalized Incremental Stress-State dependent damage Model (GISSMO) in LS-DYNA<sup>®</sup> has now been widely adopted by the automotive industry to predict metallic materials’ fracture occurrences in both forming and crashworthiness simulations. Calibration of the nominal GISSMO is typically based on material characterization data along a certain representative material orientation. Nevertheless, many rolled or extruded metallic materials, such as advanced high-strength steel (AHSS) sheets, exhibit accentuated anisotropic fracture behavior, even though, notably, some of these materials show comparatively weak anisotropic plasticity in the meantime. Accordingly, in this work, the deformation and fracture behavior of a selected AHSS grade, Q&P980 steel, was first characterized based on a series of mechanical experiments under simple shear, uniaxial tension, plane strain, and equi-biaxial tension conditions. Then, material models were calibrated based on the plasticity and fracture data. Two fracture models, either stress- or strain-based, were applied to fit the fracture loci of the target material, which then could be directly implemented into the material cards in LS-DYNA<sup>®</sup>. Particularly, to simulate the anisotropic fracture behavior of the target material, an extended GISSMO material card (eGISSMO) was introduced and highlighted in this work. Unlike the nominal GISSMO, the eGISSMO integrated the different anisotropic fracture loci and damage accumulation along three material orientations (longitudinal, diagonal, and transverse) into a single material card. In the subsequent validation based on a customized three-point-bending (3PB) testing setup on hat-section samples, only the finite element (FE) model using the calibrated eGISSMO successfully simulated the anisotropic fracture bifurcation observed in the actual experiments.</div></div>

A novel technique to measure the local mechanical properties of third generation advanced high strength steel resistance spot welds

Distinct microstructures and mechanical properties are produced in resistance spot welds (RSW) of advanced high-strength steels (AHSS). The local mechanical properties within the fusion zone (FZ) and heat-affected zones (HAZ) govern the global failure response of RSW automotive structural assemblies in crash events. In this study, a novel experimental procedure was developed to characterize the local properties of RSW sub-regions using sub-size tensile and simple shear specimens extracted from the spot weld, with local strain measurement utilizing digital image correlation (DIC) techniques. The FZ properties were obtained using a simple shear test, while the HAZ properties were characterized using a tensile test containing a central RSW. A range of AHSS grades were considered to evaluate the proposed methodology, including 3rd Gen-980, 3rd Gen-1180, and press-hardened steel (PHS1500) with respective minimum ultimate tensile strengths of 980 MPa, 1180 MPa, and 1500 MPa. Strain localization occurred wherever the hardness was lowest such as in the subcritical heat-affected zone, with limited deformation in the harder regions, such as the upper-critical HAZ and FZ. The local stress-strain responses were then compared to a hardness scaling approach to estimate the tensile properties of RSW based on the local hardness in each zone. The hardness scaling method only appears to be a viable method if the microstructures across the weld have similar microconstituents in all sub-zones across the weld.

A comprehensive evaluation of tempering kinetics on 3rd generation advanced high strength steels

Emerging third-generation advanced high strength steels (3G AHSS) grades are being increasingly considered for autobody applications owing to their complex microstructures providing an excellent combination of strength and ductility. When welded with RSW, 3G AHSS such as 3G 1180 and 3G 980, exhibits a solid-state transformation in the sub-critical-heat-affected-zone (SCHAZ) due to the heat generated during the RSW process. The resulting changes in microstructure can affect the mechanical properties of the material within and around the spot welds and thereby influence the performance of welded components in case of a vehicle crash event. To study metallurgical transformations in the SCHAZ during the RSW process, 3G 1180 and 3G 980, and a reference martensitic press hardened steel, PHS 1500, were selected. These materials were subjected to isothermal tempering experiments at temperatures ranging between 350 °C and 650 °C and different tempering times from 0.2 s to 24 h. It was shown that the Hollomon-Jaffe model accurately describes the tempering kinetics of 3G 1180 and PHS 1500 but does not predict the transformation processes in the SCHAZ of 3G 980. The PHS 1500 and 3G 1180 showed softening (tempering) for all combinations of investigated tempering times and temperatures due to the decomposition of martensite. In contrast, the 3G 980 showed a combination of softening by the formation of martensite and secondary ferrite combined with secondary hardening due to the formation of fine M2C typed plate-like carbides. Furthermore, during long tempering, the M2C particles dissolve into the matrix and were replaced by cementite particles in 3G 980.

Local buckling of cold-formed high-strength steel hollow section columns at elevated temperatures

A numerical investigation is conducted on the local buckling behavior of cold-formed square hollow section (SHS) and rectangular hollow section (RHS) columns made of normal and advanced high-strength steels at elevated temperatures. Shell finite element models validated against experimental results are used in a parametric numerical study to evaluate the local buckling strength at ambient and elevated temperatures of SHS and RHS columns fabricated with G450, dual-phase, and martensitic steels. The modeling uses recent experimental data on the elevated temperature behavior of advanced high-strength steels. The numerical results are compared with the design strengths calculated by the Direct Strength Method in AISI S100. Results show that the current method in AISI S100 consistently captures the trend but overestimates the ultimate capacity of G450 cold-formed columns both at ambient and elevated temperatures by about 10% for the SHS columns and 6% for the RHS columns. The current Direct Strength Method also yields higher estimates of capacity than the finite element models for advanced high-strength steel grades. Therefore, a modification to the method for local buckling capacity is proposed for these sections. The modified Direct Strength Method proposed herein can be used to evaluate the local buckling strength of cold-formed SHS and RHS columns made of normal and advanced high-strength steels at temperatures up to 700 °C.

Effect of external loading on liquid metal embrittlement severity during resistance spot welding

Advanced high strength steels (AHSS) used in automotive structural components are protected using zinc coatings. However, the steel/zinc system creates the potential for liquid metal embrittlement (LME) during resistance spot welding (RSW). Manufacturing conditions, such as mechanical restraint, were simulated by external loading on the sample during welding. The results showed increased LME severity in all cases, but the material response to external loading was different depending on the material’s inherent susceptibility to LME. Therefore, the manufacturing conditions are more critical for certain AHSS grades. This work provides guidelines on process design for LME mitigation and awareness during manufacturing.

Assessing the effect of the experimental parameters in the evaluation of the essential work of fracture in high-strength thin sheets

The essential work of fracture methodology (EWF) has been successfully adopted to evaluate the fracture toughness of various metals and polymers. However, some aspects of the methodology are still far less understood, such as the influence of the experimental parameters on EWF measurement in thin metal sheets. In the present paper, the ligament range criterion of the EWF approach was revised for several advanced high-strength steels (AHSS). The validity of the upper and lower ligament length limits given by the ESIS protocol is redefined and rationalized according to the necking capability and the plasticity behaviour of the different AHSS grades. The work provides a new criterion to define the minimum ligament length to be tested, based on the minimum distance required by the crack to fully develop the necking capability of the material. The width constraint is too restrictive and has no effect on the deviation from linearity in the upper range. On the other hand, the maximum ligament length is proven to be controlled by the size of the plastic zone as proposed by the ESIS protocol.

Mechanical Behavior of Multi-Phase Steels Comprising Retained Austenite.

The retained austenite (RA) in advanced high-strength steel (AHSS) grades, such as dual-phase (DP) steels, plays an important role on their formability. Thanks to the transformation-induced plasticity (TRIP) effect that occurs during the mechanically induced transformation of RA into martensite, additional ductility is obtained. Martensite has a higher flow stress than austenite; hence, the transformation results in an apparent hardening, which is beneficial for the stability of deformation. The stability of RA at a given temperature strongly depends on its carbon content, which, in AHSS, is not uniform but distributed. The aim of this study is to build a model that predicts the transformation as well as TRIP in a DP steel grade with RA. A physics-based kinetic model is presented that captures the transformation of retained austenite based on the thermodynamic driving force of the applied stress. A direct analytical estimate of transformation plasticity is provided, which is consistent with the kinetic model. Transformation kinetics is incorporated in a self-consistent, mean-field homogenization-based constitutive model. Finally, an indication of the effect of transformation of retained austenite on formability is given.

A geometrical parametric study of side door reinforced beams under lateral impact load

In this work, crash characteristics of side door reinforced beams under low speed impact were investigated. The side door panels of three different cross-sections, namely, circular, U and W shapes were considered. The advanced high strength (AHS) steel grade 1000 was used and both quasi-static and dynamic tensile tests were carried out for obtaining the rate-dependent flow stress behaviour. The geometries of all panels were modified with a specified design space constraint. FE simulations of profile variants under lateral impact load were carried out and resulting peak forces and maximum intrusions were afterwards determined. The most effective geometrical parameters for enhancing impact resistance of each reinforced beam were identified. The Wshape panel exhibited the greatest compromise of increasing crushing force efficiency, potential of weight reduction and minimizing structural damage. Finally, side door beams with combined best configurations were proposed and their crashworthiness including normalized specific energy absorptions were compared.

Optimization of Metallographic Sample Preparation for AFM/SKPFM Based Phase Distinction of Complex and Dual Phase High Strength Steels

Abstract For the comprehensive investigation of advanced high strength steel grades, like complex and dual phase steels, Atomic Force Microscopy (AFM) and Scanning Kelvin Probe Force Microscopy (SKPFM) have proven to be useful tools, especially for analysis of hydrogen permeability of the individual steel phases. However, for these studies a preparation route, exposing the microstructure of the steel, is necessary. Various sample preparation methods were examined, focusing on electropolishing and sputtering, and the selected route was optimized to guarantee reproducibility and stability of the prepared surface. Electropolishing was shown to be highly efficient to selectively reveal the individual steel phases without introducing strong topographical features disturbing the AFM measurements. A subsequent sputtering step was introduced to improve the stability and preservability of the surface up to several months. Finally, distinction of the steel phases via AFM/SKPFM, was complemented and compared with results from EBSD and XRD.

Assessment of scatter on material properties and its influence on formability in hole expansion

The plastic deformation included in the technological processes of sheet metal forming allows the production of different components with different geometries. Therefore, the formability of metallic materials is an essential feature in stamping processes. One of the inevitable scattering sources when considering these processes is the variability of material properties, which can change from coil to coil, from batch to batch and from supplier to supplier. This constitutes a significant obstacle in improving the quality of any manufacturing process. The current study investigates the formability and the mechanical characterization of a defined advanced high strength steel grade, considering different suppliers, coils, and batches in order to evaluate its influence on the results of the sheet metal forming process. A microstructure analysis is also considered to understand the impact of micro-scale variables (e.g. grain size) in macro-scale behavior. Additionally, numerical simulations were taken into consideration to evaluate the influence of mechanical characterization in the results.

Finite Element Analysis of Reinforcement Rocker Part Made from JAC780Y Steel Sheets

The manufacturing industries for automotive parts aim to develop technologies for reducing vehicle weight in order to decrease fuel consumption. However, passive safety function for drivers and passengers must not be impaired or should be even improved. Therefore, advanced high strength steel sheet plays more and more important role in designing automotive components. Nowadays, prediction of formability for sheet metal stamping has high capability more than the past. The major challenge is springback prediction. Moreover, it assists in the tooling design to correctly compensate for springback. Especially in automotive production, springback effects have been generally exhibited distinct after forming process of the high strength steel sheets. The springback effect occurred in the deformed state of metal parts must be taken into account by designing any sheet metal panels. Then, the purpose of the present research is to investigate the springback phenomenon of an automotive part named Reinforcement Rocker RL made from an advanced high strength steel grade JAC780Y, after stamping. In addition, the tools design has been carried out. Finite Element (FE) program known as DYNAFORM (based on LS-DYNA solver), has been applied to analyze and improve the springback effect on such forming part. An anisotropic material model according to type 36 (MAT_036 3-PARAMETER_BARAT) was applied. The results obtained from simulations were compared with required parts in each section. Then, the die surface from compensation in 2nd step forming was modified to use. Finally, the simulation part was verified with the real stamping part. It was found that the finite element simulation showed high capability for prediction and compensation of springback in high strength steel sheets forming.

An Overview of High Yield Strength Twinning-Induced Plasticity Steels

Twinning-induced plasticity (TWIP) steel is a second-generation advanced high strength steel grade developed for automotive applications. TWIP steels exhibit an excellent combination of strength and ductility, mainly originating from the activation of deformation twinning. However, TWIP steels generally exhibit a relatively low yield strength (YS), which limits their practical applications. Thus, developing high YS TWIP steels without ductility loss is essential to increase their industrial applications. The present work summarizes and discusses the recent progress in improving the YS of TWIP steels, in terms of precipitation strengthening, solid solution strengthening, thermomechanical processing, and novel processes. Novel processes involving sub-boundary strengthening, multi-phase structure, and gradient structure as well as the control of thermomechanical processing (recovery annealing and warm rolling) and precipitation strengthening were found to result in an excellent combination of YS and total elongation.

Effect of indentation loading type on the mechanical properties of advanced high strength steel grade DP 800

The goal of this study was to acquire mechanical properties of a dual-phase grade advanced high strength steel (AHSS) by means of different microindentation loading conditions. Conventional, cyclic, and multi-step indentations were performed on DP 800 sample; and Young’s modulus, hardness values were obtained by the depth-sensing indentation technique. Effects of different load levels (50–300 mN) and type of indentation on the results were also analyzed through ANOVA. The load levels experimented yielded overall material response rather than its constituents (e.g., martensite and ferrite). Both hardness and Young’s modulus tend to decrease with increasing maximum indentation load level, especially from 50 to 100 mN, which is regarded as indentation size effect.

Analysis of forming limit behaviour of high strength steels under non-linear strain paths using a micromechanics damage modelling

The formability characteristics of advanced high strength (AHS) steel grades 780 and 1000 subjected to varying forming histories including strain path changes were described by means of a damage modelling approach. The Marciniak tests of the examined steels were firstly carried out to apply pre-strains under various deformation modes. Afterwards, tensile, stretch-bending and hole expansion test of the pre-strained samples were performed. FE simulations coupled with the Gurson-Tvergaard-Needleman (GTN) ductile damage model were conducted to predict failure occurrences of AHS samples in the two-stage forming tests. The damage model parameters were determined under consideration of pre-strain effects on void evolution mechanisms and its interdependencies with microstructure constituents of steels, for which hybrid methods of metallographic analyses, local strain measurement and representative volume element (RVE) simulations were employed. Then, forming limits of the investigated steels in different forming procedures with non-proportional loading paths were characterized. It was shown that the micromechanics damage simulations in combination with the proposed parameter identification scheme could precisely represent the fracture states of steels under such complex forming conditions.

High Strain Rate Tensile Behavior of 1180MPa Grade Advanced High Strength Steels

<div class="section abstract"><div class="htmlview paragraph">Tensile behavior of advanced high strength steel (AHSS) grades with strengths up to 980 MPa has been extensively studied. However, limited data is found in literature on the tensile behavior of steels with tensile strengths of the order of 1180 MPa, especially at nominal strain rates up to 500/s. This paper examines tensile flow behavior to fracture of four different 1180 MPa grade steels at strain rates of 0.005/s, 0.5/s, 5/s, 50/s and 500/s using an experimental methodology that combines a servo-hydraulic tester and high speed digital image correlation. Even though the strength increase with the strain rate is consistent between the four different materials, the total elongation increase with the strain rate varies widely. Some insights as to why this occurs from examination of the steel microstructure and variation of retained austenite with strain are offered.</div></div>

Bake Hardening Behavior of DP, TBF, and PHS Steels with Ultimate Tensile Strengths Exceeding 1 GPa

<div class="section abstract"><div class="htmlview paragraph">Third generation advanced high strength steels (AHSS) have been developed combining high strength and formability, allowing for lightweighting of vehicle structural components. These AHSS components are exposed to paint baking operations ranging in time and temperature to cure the applied paint. The paint baking treatment, combined with straining induced from part forming, may lead to increased in-service component performance due to a strengthening mechanism known as <i>bake hardening.</i> This study aims to quantify the bake hardening behavior of select AHSS grades. Materials investigated were press hardenable steels (PHS) 1500 and 2000; transformation induced plasticity (TRIP) aided bainitic ferrite (TBF) 1000 and 1200; and dual phase (DP) 1000. The number designations of these grades refer to minimum as-received ultimate tensile strengths in MPa. Paint baking was simulated using industrially relevant times and temperatures from 15 to 60 min and 120 to 200 °C, respectively. Samples were prestrained 0, 2, or 5 pct to replicate part forming. Bake hardening values ranging from 90 to 140 MPa were observed for DP and TBF grades that were prestrained 2 pct and baked at 170 °C for 20 min. However, ductility diminished for these steels when subjected to 5 pct prestrain with uniform elongations after baking decreasing to 1 pct in some instances. PHS steels, on the other hand, showed substantial increases in yield strength without prestrain. Increases of 122 and 175 MPa were recorded for PHS 1500 and PHS 2000, respectively, following baking at 160 °C for 60 min. However, ultimate tensile strengths decreased due to reduced strain hardening, while total elongations decreased slightly.</div></div>

Development of advanced high strength steels using hydrogen quench continuous annealing technology

By using hydrogen quench continuous annealing technology, Stelco Inc. has developed a suite of Advanced High Strength Steel (AHSS) grades with tensile strength greater than 1000 MPa to meet standard automotive specifications and for unique customer requirements. These grades were optimized by correlating chemical composition and processing parameters with microstructures and mechanical properties. Dual-Phase 980 (Stelco trademarked STELMAXTM 980 DP), Multi-Phase 1180 (STELMAXTM 1180 MP), Martensitic Steel 1300 (STELMAXTM 1300 M) and 1500 (STELMAXTM 1500 M) products met strength and formability requirements with excellent flatness and surface quality. Hydrogen quench continuous annealing technology not only ensures all developed AHSS grades have consistent mechanical properties across the entire strip length (from strip head to tail) and width (from edge to edge), but also produces high product yield compared with other continuous annealing processes.