Fatigue properties of transformation-induced plasticity and dual-phase steels for auto-body lightweight: Experiment,modeling and application

The substitution of conventional high strength steels (HSS) with advanced high strength steels (AHSS), e.g., low-alloy multiphase transformation-induced plasticity steel (TRIP steel) or dual-phase steel (DP steel), for body lightweight brings about increased stress of notched components. Thus the fatigue properties of TRIP and DP steels and the fatigue life of notched lightweight design are important considerations for reasonable material selection during the design stage of auto-body. For the mentioned issue, cyclic strain-controlled fatigue properties of TRIP and DP steels with equivalent grade and lightweight result were investigated experimentally. Different cyclic behaviors of TRIP and DP steels were observed due to different interior microstructures. The cyclic stress behavior of TRIP steel is characterized by cyclic hardening followed by stable at lower strain amplitudes, and softening at higher strain amplitudes; however cyclic softening followed by stable occurs consistently for DP steel throughout entire strain amplitude range of test. TRIP steel possesses enhanced fatigue life and cyclic stress at the same strain amplitude than DP steel. Furthermore, local strain-life models of two steels were developed by linear regression of experimental data, to predict and compare the fatigue life of notched body structures made of them by finite element method. The simulation result illustrates that TRIP steel can provide more beneficial potential than DP steel for the lightweight design of notched body structures from the viewpoint of fatigue resistance.     

Press hardening of alternative materials: conventional high- strength steels

The increase in strength of new high strength steels (HHS) and advanced high strength steels (AHHS) has led to forming issues, such as high springback, low formability, increase of forming forces and tool wear. These problems increase thecosts of manufacturing and maintaining stamping tools in the automotive industry. The aim of this research was to analyse the advantages of applying the press- hardening process toconventional HSS and AHSS steel to increase their formability and therefore reduce the number of forming steps and productioncosts. With this aim in mind, the press-hardening process was used to manufacture an industrialcomponent using four different automotive steel grades: dual phase (DP),complex phase (CP), transformation- induced plasticity (TRIP) and martensitic (MS) grade. Springback measurements werecarried out, together with an analysis of the obtained final mechanical properties and microstructures. The results showed that the formability of all the materials increased. The mechanical properties of theCP800 and TRIP700 materials were maintained or even improved, whereas those of the MS1200 and HCT980X materials were significantly reduced. Weconclude that press hardening is a suitable manufacturing process forCP800 and TRIP700components.     

超高强QP980钢冲压成形性能

目的研究宝钢新一代先进高强钢QP980与第一代先进高强钢DP980的冲压成形性能。方法采用单向拉伸、光学应变分析和帽型拉弯成形等试验方法,分析两种超高强钢在加工硬化、成形极限、拉弯成形侧壁减薄和回弹等性能特性。结果 QP980的伸长率达到21.6%,与DP980钢相比有更高的强塑积,在变形过程中能够维持较高的瞬时n值,进而提高了材料的成形极限。帽型拉弯试验中,在不同的流入距离条件下,QP980钢的侧壁减薄率均低于DP980钢。在不同压边力条件下,QP980(厚度1.0 mm)材料的回弹量大于DP980(厚度1.2 mm)材料。结论 QP980在梁型件成形过程中具有优良的抗减薄特性,采用QP980钢进行冲压成形时应考虑比同级别DP980钢更大的回弹补偿量,或者采用更大变形量的工艺设计。     

A Robust Inverse Method Based on Least Square Support Vector Regression for Johnson-cook Material Parameters

The purpose of this study is to propose a robust inverse method for estimating Johnson-Cook material parameters. The method is shown through illustrative examples for two different advanced high strength steel (AHSS) materials (DP980 and TRIP780) using set of data from impact experiments with different velocities. Compared with widely mixed numerical experimental methods, the suggested inverse method has the capability to guarantee the robustness of the obtained parameters by considering uncertainties. The inverse problem is converted into multi-objective optimization problems. Furthermore, in order to improve the performance in efficiency and accuracy, metamodeling techniques and global optimization method are integrated. The final results demonstrate that the experimental and simulation curves are well matched based on identified by the suggested robust inverse method.     

Dual phase versus TRIP strip steels: Microstructural changes as a consequence of quasi-static and dynamic tensile testing

The development of present day and future vehicles is being driven by the need to simultaneously reduce mass and increase passenger and pedestrian safety. For this reason, the steel industry has developed strip steel grades with suitable properties, as required for meeting the demands placed on the automotive manufacturers. Two of these strip steel grades are the Dual Phase (DP) and the Transformation Induced Plasticity (TRIP) steels, which are thought to offer solutions for critical crash component criteria. Limited published information is available on the changes in microstructure of these novel strip steel grades at different rates of deformation.This paper examines the change in microstructure of a range of both commercial and experimental DP and TRIP strip steel grades, which were tensile tested at low (0.001 s^− 1) and very high strain rates (200 s^− 1). The DP and TRIP microstructures were characterised in terms of ferrite grain size, ferrite grain elongation and volume fraction of constituent phases. The specimens were examined following deformation and compared to the as-received condition to assess microstructural changes.This paper concentrates only on microstructural changes through dynamic tensile testing of DP and TRIP grades at low and high strain rates. The full crash performance data from the dynamic tensile tests and crushing of box sections is presented in a separate publication. [S. Oliver, G. Fourlaris and T.B. Jones, ‘Dual Phase versus TRIP strip steels: a comparison of dynamic properties for automotive crash performance’, Materials Science and Technology, 2006, (submitted for publication)].     

先进高强度钢板弯曲类回弹特性的试验研究

随着先进高强钢板在汽车及航天航空领域的广泛应用,回弹导致的成形精度问题日益突出.为了获取先进高强钢的弯曲回弹特性,通过采用U形件回弹模型,针对600MPa级别的3种典型高强钢(DP钢、TRIP钢、HSLA钢)进行了回弹试验研究.实验结果表明:在相同变形条件下,TRIP钢弯曲回弹最大,DP钢次之,HSLA的弯曲回弹最小;不同工艺条件、不同材料性能参数对弯曲回弹呈单调的影响规律,而润滑条件对弯曲回弹的影响趋势并未出现一致性规律.     

Tensile properties of fiber laser welded joints of high strength low alloy and dual-phase steels at warm and low temperatures

High strength low alloy (HSLA) and dual-phase DP980 (UTS ⩾ 980 MPa) steels were joined using fiber laser welding in similar and dissimilar materials combinations. The welded joints were characterized with respect to microhardness and tensile properties at three different temperatures: −40 °C, 25 °C, and 180 °C. Tensile properties of the welded joints were compared to those of the base metal (BM) obtained under similar conditions. A good correlation was found between the welded joints and the BM in relation to the tensile properties obtained at the different temperatures. A general trend of increase in the yield strength (YS), the ultimate tensile strength (UTS) and energy absorption (EA) with decreasing temperature was observed; however, work hardening coefficient was not altered and insignificant scatter was observed in case of the elongation. However, in the DP980 steel, dynamic strain ageing was observed only in the BM.     

Experimental and theoretical formability analysis using strain and stress based forming limit diagram for advanced high strength steels

In this study, experimental and numerical analyses of Forming Limit Diagram (FLD) and Forming Limit Stress Diagram (FLSD) for two Advanced High Strength Steel (AHSS) sheets grade DP780 and TRIP780 were performed. Initially, the forming limit curves were experimentally determined by means of the Nakazima forming test. Subsequently, analytical calculations of both FLD and FLSD were carried out based on the Marciniak–Kuczinsky (M–K) model. Additionally, the FLSDs were calculated using the experimental FLD data for both investigated steels. Different yield criteria, namely, von Mises, Hill’s 48, and Barlat2000 (Yld2000-2d) were applied for describing plastic flow behavior of the AHS steels. Both Swift and modified Voce strain hardening laws were taken into account. Hereby, influences of the constitutive yield models on the numerically determined FLDs and FLSDs were studied regarding to those resulted from the experimental data. The obtained stress based forming limits were significantly affected by the yield criterion and hardening model. It was found that the forming limit curves calculated by the combination of the Yld2000-2d yield criterion and Swift hardening law were in better agreement with the experimental curves. Finally, hole expansion tests were conducted in order to verify the different failure criteria. It was shown that the stress based forming limit curves could more precisely describe the formability behavior of both high strength steel sheets than the strain based forming limit curves.     

Experimental study on high strain rate behavior of high strength 600–1000 MPa dual phase steels and 1200 MPa fully martensitic steels

As one of high grade advanced high strength steels (AHSSs), dual phase (DP) steel sheets and fully martensitic (MS) steel sheets have been successfully used in automotive crash-resistance components for its great benefit in reducing vehicle weight while improving car safety as well as their advantage in cost saving through cold forming instead of hot forming. The strain rate sensitivity of 600/800/1000 MPa DP and 1200 MPa MS were studied in this paper through a split Hopkinson tensile bar (SHTB) setup and compared with each other. The experiments showed that all dual phase (DP) AHSS ranging from 600 MPa to 1000 MPa are of positive strain rate sensitivity. While for the tested 1200 MPa MS, negative strain rate sensitivity has been found. Possible reason for the difference has been investigated through metallographical observation and their microstructures.     

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

Advanced High Strength Steels (AHSS) is a promising material for automotive applications due to its high strength-to-weight ratio compared to other steels. Recently third generation steels have been developed which show intermediate properties between first and second generation AHSS. Formability analysis was performed between first generation Transformation Induced Plasticity (TRIP) and second generation Quenched and Partitioned (Q&P) AHSS. The main objective of the study is to perform formability analysis of TRIP and Q&P AHSS. The chemical compositions of both the steels are almost similar but different processing conditions lead to microstructural variations. Experimental and simulated strain-path diagram (SPD) was plotted from drawing to stretching regions using Limit Dome Height (LDH) test and Finite Element Method (FEM) respectively. The formability of TRIP steel is higher when compared to Q&P steels. Stretching regions show large deviation between experimental and simulated SPD for both the steels. A new strain localization criterion is proposed to construct a forming limit curve (FLC) for both experimental and simulated SPD. The proposed failure criterion is compared with other failure criteria for FLC prediction. The FLC based on new strain localization criterion shows better agreement with experimental FLC compared to other failure criteria.     

Characterisation of formability behaviour of multiphase steels by micromechanical modelling

Multiphase steels are new Advanced High Strength Steels (AHSS), which have been developed for the automobile industry for the purpose of reducing of car body weight. These steels offer an excellent combination of high strength and large elongation due to the coexistence of harder and softer phases in their microstructure. The advantageous properties of multiphase steels can be utilised by regulating the type, amount, formation and spatial distribution of the different constituent phases. To describe the influences of the heterogeneous microstructure on the mechanical properties and the complex fracture mechanisms, an approach is presented using Representative Volume Elements (RVEs) on a micro level. Three dimensional RVE simulations were conducted under considerations of metallographic analysis for a Dual Phase (DP) steel and a Transformation-Induced Plasticity (TRIP) steel. The Gurson–Tvergaard–Needleman (GTN) damage model was applied to investigate the local crack initiation in steel sheets during various forming processes. In conclusion, a failure prediction based on microstructure is proposed for the material characterisation in sheet metal forming of multiphase steels.     

Advanced cold rolled steels for automotive applications

Advanced multiphase steels offer a great potential for bodies‐in‐white through their combination of formability and achievable component strength levels. They are first choice for strength and crash‐relevant parts of challenging geometry. The intensive development of high‐strength multiphase steels by ThyssenKrupp has led to hot dip galvanizing concepts with an outstanding forming potential. Hot rolled, hot dip galvanized complex phase steels are currently produced in addition to cold rolled DP and RA steels. New continuously annealed grades with tensile strength levels of up to 1000 MPa in combination with sufficient ductility for applications mainly in the field of structural automobile elements make use of the classic advantages of microalloying as well as the principles of DP and TRIP steels. Further improvement of properties will be reached by the new class of high manganese alloyed steels.     

An investigation into the mechanical behavior of a new transformation-twinning induced plasticity steel

In recent years, the transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) steels have been the focus of great attention thanks to their excellent tensile strength-ductility combination. Accordingly the mechanical behavior of an advanced microalloyed TRIP–TWIP steel, the compression tests were conducted from 25 to 1000 °C. This experimental steel shows a high compressive strength of 1280 MPa with the yield strength of 385 MPa as well as an outstanding strain hardening rate of about 14,000 MPa at the 25 °C. In addition the results indicate that the plastic deformation in the range of 25–150 °C is controlled by both the strain-induced martensite formation and mechanical twinning. However the mechanical twinning has been speculated as the only deformation mechanism in the temperature range of 150–1000 °C. This as well has led to an outstanding grain refinement via grain partitioning. The occurrence of mechanical twinning at such high temperatures is a novel observation in this grade of TRIP–TWIP high manganese steels.     

Springback simulation of advanced high strength steels considering nonlinear elastic unloading–reloading behavior

The stress–strain response of some materials, such as advanced high strength steels, during unloading is nonlinear after the material has been loaded into the plastic deformation region. Upon reloading, the response shows a nonlinear elastic response that is different from that in unloading. Therefore, unloading–reloading of these materials forms a hysteresis loop in the elastic region. The Quasi-plastic–elastic model (Sun and Wagoner, 2011) was modified and combined with both isotropic-nonlinear kinematic hardening and two-surface plasticity models to simultaneously describe the nonlinear unloading response and complex cyclic response of sheet metals in the plastic region. The model was implemented as user-defined material subroutines, i.e. UMAT and VUMAT, for ABAQUS/Standard and ABAQUS/Explicit finite element codes, respectively. Uniaxial loading-unloading tests were performed on three common grades of automotive sheet steel: DP600, DP980 and TRIP780 steel. The model was verified by comparing the predicted material response with the corresponding experimental response. Finally, the model was used to predict the springback of a U-shape channel section formed in a plane-strain channel draw process. The results showed that the model was able to considerably improve springback predictions compared to the usual assumption of linear elastic unloading.     

Nd:YAG laser butt welding of a 12 vol.% SiC particulate-reinforced magnesium alloy composite

A renovated shielding gas apparatus was successfully used to protect the laser-welded pool during Nd:YAG laser welding process of a particulate SiC-reinforced magnesium alloy composite. Laser welding parameters, such as beam diameter and scanning velocity, were optimized to obtain sound welded seam. The microstructure and elemental distribution of the laser-welded seam were analyzed by employing scanning electron microscope incorporating energy dispersive X-ray. The strength of the seams was measured by tensile test. Results showed that: an increase in laser beam diameter from 1.0 mm to 1.5 mm effectively avoided severe burning and evaporation; sound laser-welded seam was produced by suitably choosing a laser scanning velocity of 150 mm min^− 1 while V-shaped and the inadequate penetrative seams were generated at lower and higher scanning velocity respectively; the microstructure of the seams was nearly the same as that of the original substrate; the tensile strength of the seams reached about 88 MPa.     

Analysis of stamping performances of dual phase steels: A multi-objective approach to reduce springback and thinning failure

The industrial interest on light weight components has contributed in the last years to a significant research effort on new materials able to guarantee high mechanical properties, good formability and reasonable costs together with reduced weights when compared to traditional mild steels. Among such materials advanced high strength steels (AHSS) such as transformations induced plasticity (TRIP) and dual phase (DP), and light weight alloys proved their usefulness in stamping of automotive components. As AHSS are concerned, one of the main drawbacks is related to springback occurrence. Many aspects have to be taken into account when springback reduction is investigated: material behavior issues, process conditions, numerical simulations parameters calibration, geometrical aspects and so on. Moreover, springback minimization problems are typically multi-objective ones because springback reduction may conflict with other goals in stamping design such as thinning reduction. In this paper, such problem was investigated through integration between numerical simulations, Response Surface Methodology and Pareto optimal solutions search techniques. The design of a U-channel stamping operation was investigated utilizing two different dual phase steel grades: DP 1000 and DP 600. An explicit/forming-implicit/springback approach was utilized for the numerical simulations. Friction conditions and blank holder force were optimized as design variables in order to accomplish two different objectives: reduce excessive thinning and avoid excessive geometrical distortions due to springback occurrence.     

980 MPa先进高强钢材料在门槛加强梁上的辊压应用

目的为获得980 MPa级别不同先进高强钢材料在辊压零件上应用时的差异,研究典型980 MPa先进高强钢的辊压成形特性。方法采用调研和统计方法获得门槛辊压用材的主流强度与钢种,基于三点弯曲的实验方法,获得了980 MPa先进高强钢材料在指定角度和弯曲半径下的回弹特性,针对典型的门槛加强梁零件,采用CAE方法评估材料应用的可行性。结果门槛零件980 MPa级别所采用的主要先进刚强钢钢种为双相钢(DP)和马氏体钢(MS),在相对弯曲半径比R/t=2.5的情况下,980DP回弹角度最小,980QP次之,980MS最大;早期模具回弹补偿设计时对于90°圆角可给予11°~14°的回弹过弯角度;成形过程中有可能出现边波缺陷,需优化辊压成形变形过程。结论 980 MPa先进高强钢材料在应用于辊压零件时,应在设计阶段充分考虑材料回弹特性,同时变形过程应合理,避免产生边波等质量缺陷。     

马氏体含量对合金化热镀锌双相钢电阻点焊接头组织与性能的影响

目的 研究在双相钢电阻电焊过程中马氏体含量对点焊接头组织、性能的影响规律。方法 使用电阻点焊机对DP780、DP980、DP1180 3种马氏体含量不同的锌铁合金化热镀锌双相钢进行焊接,利用欧姆表、光学显微镜、扫描电镜、拉伸机和显微硬度计等设备,对基板的电阻率、工艺窗口、接头力学性能、焊点断裂模式、金相组织进行表征。结果 在AWS D8.9M-2012焊接标准体系下,DP780、DP980、DP1180焊接电流窗口依次减小,DP780、DP980、DP1180 3种材料在最大焊接电流下的焊核直径基本一致;熔核区硬度呈增大趋势,DP780点焊接头软化不明显,DP980和DP1180的热影响区出现明显的软化现象,这主要是由母材热影响区中的马氏体回火造成的。DP780、DP980、DP1180的最大剪切力分别为23 062、27 317、28 183 N。DP780为拔核断裂模式,DP980和DP1180为部分拔核断裂模式。结论 双相钢中马氏体含量的增加会使焊接电流窗口降低,整体向焊接电流减小的方向偏移,但是会提高上限电流的焊点承载强度。     

Hybrid laser/arc welding of advanced high strength steel to aluminum alloy by using structural transition insert

The present investigation is related to the development of the welding procedure of the hybrid laser/arc welding (HLAW) in joining thick dissimilar materials. The HLAW was applied to join aluminum alloy (AA6061) to an advanced high strength steel (AHSS) where an explosively welded transition joint, TRICLAD®, was used as an intermediate structural insert between the thick plates of the aluminum alloy and AHSS. The welds were characterized by an optical microscope, scanning electron microscope (SEM), tensile test, charged coupled device (CCD) camera, and microhardness measurement. The groove angle was optimized for the welding process based on the allowed amount of heat input along the TRICLAD® interface generated by an explosive welding. The weld was fractured in the heat affected zone of the aluminum side in the tensile test. The microhardness was shown that the temperature variation caused minor softening in the heat affected zone satisfying the requirement that the width of the softened heat affected zone in the steel side falls within 15.9 mm far away from the weld centerline. The microstructure analysis showed the presence of tempered martensite at the vicinity of the weld area, which it was a cause of softening in the heat affected zone.     

Low energy O2 and N2O ion beam modification of polyimide for improving adhesive strength of Cu/PI in roll-to-roll process

To enhance the weak mechanical durability of directly deposited copper layers on polyimide (PI) film due to their poor adhesive strength, a continuous roll-to-roll process involving surface modification using a reactive ion beam irradiation and in-situ deposition process is studied. The polyimide film is modified by an ion source with a linear stationary plasma thruster (LSPT) in the vacuum roll-to-roll process. An O₂ ion beam, with beam energy of 214 eV and beam current density of 0.78 mA/cm², and N₂O ion beam, with 220 eV and 0.69 mA/cm², irradiate PI film in winding speed of 0.5 m/min. The surface energy increases from 38 mN/m for the pristine PI film to 80 mN/m after beam irradiation at an ion fluence of 3.5 × 10¹⁶ ions/s. After beam irradiation, a 10 nm thick tie layer and 200 nm thick copper layer are successively deposited by in-situ DC magnetron web coating. The peel strength of the copper layer on the PI film is enhanced from 0.4 kgf/cm without ion beam treatment to 0.71 kgf/cm after O₂ beam treatment and to 0.75 kgf/cm after N₂O beam treatment. This enhancement is closely related to the increase in the polar force originating from the formation of hydrophilic CO (carbonyl) groups on the modified PI surface.