The limit drawing ratio and formability prediction of advanced high strength dual-phase steels

Dual phase (DP) steels, being among advanced high-strength steels (AHSS), have been successfully used in the sheet metal stamping of automotive components for its great benefit in reducing vehicle weight while improving car safety. In their practical application, one of the major challenges is related to formability prediction of onset crack. This paper first conducts limiting drawing ratio (LDR) experiments to identify the maximum blank diameter of SPFC340, DP600, DP800 and DP1000 with onset crack. The fracture modes of these four types of blanks are then compared and classified into two categories: necking crack and shear crack. Further for DP1000, appropriate hardening formula is determined to fit the flow curve derived by the tensile test. Three yielding models (Hill-48, Batlat-89 and Banabic-2005) are compared with each other in the numerical simulation of DP1000 LDR onset crack. The investigation shows that a Swift and Hockett–Sherby combined formula is in good agreement with the flow curve of the tensile test and Batlat-89 yield model successfully predicts the onset shear crack of DP AHSS.     

Evolution of elastic modulus in roll forming

Roll forming is a continuous process in which a flat strip is incrementally bent to a desired profile. This process is increasingly used in automotive industry to form High Strength Steel (HSS) and Advanced High Strength Steel (AHSS) for structural components. Because of the large variety of applications of roll forming in the industry, Finite Element Analysis (FEA) is increasingly employed for roll forming process design. Formability and springback are two major concerns in the roll forming AHSS materials. Previous studies have shown that the elastic modulus (Young’s modulus) of AHSS materials can change when the material undergoes plastic deformation and the main goal of this study is to investigate the effect of a change in elastic modulus during forming on springback in roll forming. FEA has been applied for the roll forming simulation of a V-section using material data determined by experimental loading-unloading tests performed on mild, XF400, and DP780 steel. The results show that the reduction of the elastic modulus with pre-strain significantly influences springback in the roll forming of high strength steel while its effect is less when a softer steel is formed.     

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.     

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.     

先进高强度双相钢弯扭复合回弹评价与试验研究

目的 针对先进高强度双相钢冲压成形后回弹量大且多种回弹形式相互作用的问题,提出弯曲–扭曲复合回弹试验评价指标与测试方法,研究双相钢强度级别和试验条件对弯扭回弹的影响规律。方法 建立板料回弹前后的几何关系,提出弯扭复合回弹评价指标,通过改变三点弯曲试验上压杆的偏转角度,开展弯扭复合回弹试验。结果 基于合理的几何关系假设提出的评价指标适用于弯扭复合回弹试验结果分析。当上压杆偏转角度为0°时,强度越高,板料弯曲回弹越明显,回弹后横截面顶点距离和2条对角线长度变化越小;随着上压杆偏转角度的增加,当上压杆偏转0°~15°时,不同强度的3种试件(DP600、DP780、DP980)的横截面顶点距离和2条对角线长度增大,其中DP600试件的横截面顶点距离变化最小;当上压杆偏转15°~45°时,3种试件共同表现出横截面顶点距离减小、一条对角线长度减小、另一条对角线长度增加,其中DP980试件的对角线长度之差最小。结论 试验数据与分析结果表明,调整三点弯曲试验中上压杆的偏转角度可以诱发不同的扭曲回弹量,进而有效控制弯曲回弹与扭曲回弹的复合程度。分别利用板料横截面顶点距离变化及2条对角线长度变化评价弯曲回弹和扭曲回弹程度,利用二者综合评价弯扭复合回弹程度。板料的扭曲回弹受板料强度制约进而影响弯曲回弹程度。     

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.     

Crash response of advanced high-strength steel tubes: Experiment and model

The performance of non-hydroformed and hydroformed structural steel tubes in component-level crash testing was investigated using both experimental and analytical techniques. In particular, the focus was on high-strength steels that may have potential to enhance crashworthiness of automobiles. Monolithic tubes made from multiple materials and wall thicknesses were considered in this study. The following materials were used: conventional drawing quality (DDQ) steels; high-strength low alloy (HSLA-350) steels; and advanced high-strength steel (AHSS) materials comprising the dual phase alloys DP600 and DP780. The goal of this research was to study the interaction between the forming and crash response of these materials in order to evaluate their potential for use in vehicle design for crashworthiness. The tubes were hydroformed using two methods known as low- and high-pressure processes. Material characterization of all materials was carried out through quasi-static and high strain rate tensile tests in the range of 0.00333–1500 s⁻¹, and rate sensitive constitutive models for all materials were developed. The nonlinear explicit dynamic finite element code LS-DYNA, in conjunction with the validated constitutive models, was used to simulate both the hydroforming processes and the crash tests performed on the tubes. The energy absorption characteristics of the different tubes were calculated and the results from the numerical analyses were compared against the experimental data. This comparison was performed in order to determine whether the interactions between forming and crush could be adequately predicted using finite element analysis. The effects of thickness changes, work hardening, and component geometry, which resulted from hydroforming, on the crash response were also investigated. A study of the significance of strain rate and the importance of performing detailed material characterization on the accuracy of the numerical analysis was performed. Also, a parametric study on the effect of transferring forming history data between simulations on the accuracy of the numerical analysis was performed, and the importance of carrying forward the histories between multiple forming simulations was demonstrated.     

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。     

An experimental analysis of drawing characteristics of a dual-phase steel through a round drawbead

The sheet drawing characteristics of a dual-phase steel (DP600) through a round drawbead are determined experimentally using strip drawing tests. For this purpose, a drawbead simulator is designed and integrated with a standard tensile testing machine. Drawing tests are conducted with steel strips cut from 1 mm thickness blanks. The strip geometries, thickness strains, pulling forces and clamping forces are measured during drawing through a round drawbead of 5 mm bead and shoulder radius. The drawbead force parameters and thinning strains are determined from measurements. The experiments are repeated with conventional draw-quality sheets (DC06) of the same thickness for the purpose of establishing a benchmark database. A comparison of calculated drawing characteristics between two types of steels indicates the significant differences in terms of drawbead force parameters. In addition, analysis of experimental data demonstrated bead penetration, clamping force and material flow stress as the dominant factors on drawbead restraint force and blank thinning deformation for both materials. The results of experimental analyzes for both steel types can employed as model input curves for equivalent drawbead models in FE process simulations.     

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.     

超高强度钢DP1000液压成形A柱的关键技术

目的研究超高强度钢DP1000液压成形A柱仿真技术和试验效果,解决批量生产中DP100焊管技术和同步冲孔等关键技术。方法基于Autoform有限元模拟软件,仿真分析了DP1000液压成形A柱的可行性,试验了DP1000液压成形A柱零件。通过对比高频焊管和激光焊管的液压成形零件焊缝质量,解决量产零件开裂问题,开展了DP1000液压成形A柱的Cr12MoV、ASP30和SKH51不同材料的同步冲孔效果,解决DP1000材料冲头寿命和冲孔质量问题。结果采用DP1000材料可以获得液压成形A柱,具有制造可行性。与高频焊管相比,具有较小的热影响区和硬度变化较小的激光焊管,更适合DP1000液压成形A柱使用。结论 ASP30和SKH51比传统的Cr12MoV更适合于DP1000材料液压成形A柱同步冲孔,冲头寿命提高80多倍,冲孔质量良好。     

基于Yoshida-Uemori材料模型的汽车结构件冲压回弹分析

Yoshida-Uemori随动硬化材料模型能够准确描述应变路径发生变化时材料性能的改变,从而较好地反映复杂加载情况下材料的各向异性.本文基于JSTAMP件分别采用Yoshida-Uemori随动硬化材料模型和各向同性硬化材料模型对汽车高强钢结构件的冲压成形进行了仿真分析与回弹预测,研究了不同材料硬化模型对回弹预测精度...     

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。     

Effect of slide motion on springback in 2-D draw bending for AHSS

The springback behavior of two advanced high strength steel (AHSS) grades, DP 780 and DP 980, after different forming conditions, was investigated. 2-D draw bending experiments were performed using a direct-drive digital servo-press in three operation modes, conventional (V-mode), relaxation mode (Stepwise and U-mode) and the attach-detach (A-D mode). Numerical simulations were conducted to in an attempt to reproduce the results and to perform parametric studies. The material behavior was captured using the homogeneous anisotropic hardening (HAH) distortional plasticity approach together with the chord elastic modulus model. In addition, the stress relaxation effects were implemented in the code by using a creep law in order to study the influence of a stepwise slide motion mode as well as holding at the bottom dead center. Both experimental and finite element (FE) simulation results demonstrate that detachment of tools from the work-piece was effective to reduce the springback while the effect of stress relaxation was insignificant. The numerical analysis was validated and successfully explained the importance of a forming path change on the final stress state. Based on the result of this study, a new method to reduce springback was introduced.     

On twist springback in advanced high-strength steels

The sheet metal components made of advanced high-strength steel (AHSS) become fairly attractive in reducing weight and enhancing operational performance of products. However the corresponding forming process often generates more severe springback. This paper aims to investigate the behavior of twist springback in advanced high strength sheet components, where a twist rail was considered and the corresponding die and measurement tool were developed. Finite element model of the twist rail was first validated through a try-out test and then is used to carry out a parametric study on the twist springback. The results lead to an in-depth understanding of how the design and process parameters, such as transition ratio of cross-section width, corner angle, drawbead depth and material strength grade, affect the twist springback, thereby providing some insights into sensitivity analysis for the design of products and corresponding processes.     

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.     

Acoustic emissions during tensile test of DC06 and HCT600X

In this paper the acoustic emission behaviour during tensile tests of the materials DC06 and HCT600X is studied. Two steels with different characteristics (mild deep‐drawing steel DC06 and high‐strength steel HCT600X) are consciously chosen to show the influence of the material properties on the generated acoustic emissions. The acoustic emission behaviour and the corresponding signals differ clearly from each other. In addition, the effect of the strain rate as well as the rolling direction (0°, 90°) on the acoustic emission behaviour is investigated. Both parameters have a significant influence on the resulting acoustic emissions during tensile deformation. Furthermore, a new criterion based on the acoustic emission parameter FCOG (centroid frequency) for detection of damage beginning in dual‐phase steels is developed. The criterion supports the assumption that during tensile deformation of dual‐phase steels two failure mechanisms, ferrite/martensite interface decohesion and martensite phase fracture, exist.     

Springback prediction in sheet metal forming of high strength steels

Both increased weight reduction and improved passive safety have been simultaneously required for components of new vehicle generation. Thus, advanced high strength Dual Phase (DP) steels have been progressively used when making automotive parts. During each sheet metal forming process the high strength steels exhibit distinct springback effect, which is governed by strain recovery of material after load removal. The springback is variably sensitive to materials and process parameters. Considering springback occurred in a formed part is significant for designing tools and dies. In this work, both experiments and Finite Element Analyses (FEA) of a U-shape forming test were performed and compared for investigating the springback effect. Two DP steels (JSC590R and JSC780Y) with different strengths and a mild steel (JSC270C) were taken into account. The planar anisotropic material model according to Hill’s 1948, Barlat’s yield 2000, and Yoshida–Uemori kinematic hardening model were applied in the simulations. Various mechanical testing as hydraulic bulge test, disk compression test, and in particular cyclic test under tension and compression load were carried out in order to determine required materials parameters of the models. Obviously, steel with higher yield and tensile strength definitely showed an increasing in magnitude of both springback and curling. All presented material models restricted ability to predict springback effect of the examined steels, although the Yoshida–Uemori criterion provided more accurate results than other ones. The model is therefore preferred for describing the strain recovery mechanism of high strength steels, while parameter determination plays a decisive role. The cyclic test was verified to successfully describe the kinematic behaviour of material.     

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.