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Developing lightweight, stiff and crash-resistant vehicle body structures requires a balance between part geometry and material properties. High strength materials suitable for crash resistance impose geometry limitations on depth of draw, radii and wall angles that reduce geometric efficiency. The introduction of 3rd generation Advanced High Strength Steels (AHSS) can potentially change the relationship between strength and geometry and enable simultaneous improvements in both. This paper will demonstrate applicability of 3rd generation AHSS with higher strength and ductility to replace the 780 MPa Dual Phase steel in a sill reinforcement on the current Jeep Cherokee. The focus will be on formability, beginning with virtual simulation and continuing through a demonstration run on the current production stamping tools and press.

Advanced high-strength steels (AHSS) are a class of steels that have a minimum tensile strength of 500 MPa. The advantages of AHSS include superior formability and better crash energy absorption compared with conventional low-strength steels having a minimum tensile strength of 270 MPa. Several steels with a minimum tensile strength of 590 MPa have already found use in current vehicles, and others with minimum tensile strength up to 980 MPa have been qualified for use in future vehicle models. Two 780 MPa steels of interest are 780 DP (Dual Phase) and 780 TRIP (TRansformation Induced Plasticity). In this study, an examination was undertaken to compare the resistance spot-welding behavior of commercially produced 1.6 mm-thick, hot-dipped galvannealed, 780 MPa DP and TRIP steel sheet. Included in the study were evaluations of the weld lobes, weld microhardness, and the shear- and cross-tension strengths of resistance spot welds for the two steels.

A novel performance classification system has been developed for advanced high-strength steel (AHSS). This system considers intrinsic global and local formability parameters derived from standard uniaxial tension tests and is applicable to all current and future AHSS materials. The overall AHSS performance index (P.I.) is defined herein as the product of the ultimate tensile strength (UTS) and the formability index (F.I.), where F.I. is an intermediate strain value between the true uniform strain and the true fracture strain (TFS). Target P.I. values are defined for First Generation AHSS (GEN1), Improved First Generation AHSS (GEN1+), Third Generation AHSS (GEN3), and AHSS Future. Performance is further distinguished by local, balanced, and global formability characteristics and by relative yield strength (yield-to-tensile ratio). Additionally, the influence of tension test specimen geometry and fracture area measurement method on the TFS value was explored.

This paper describes static and fatigue behavior of resistance spot welds with the stack-up of conventional mild and advanced high strength steels, with and without adhesive, based on a set of lap shear and coach peel coupon tests. The coupons were fabricated following specified spot welding and adhesive schedules. The effects of similar and dissimilar steel grade sheet combinations in the joint configuration have been taken into account. Tensile strength of the steels used for the coupons, both as-received and after baked, and cross-section microstructure photographs are included. The spot weld SN relations between this study and the study by Auto/Steel Partnership are compared and discussed.

A typical forming operation of chassis components (control arms, cross members, etc.) often involves edge stretching and/or hole expansion. As a result, the edge split is a common forming failure mode. To overcome this problem, Japanese and European automakers use stretch flange high strength (SFHS) steel due to its high strength and excellent edge stretch capability. Recently, SFHS steel has gained greater attention in North America and is currently being used for upper and lower control arm applications. This paper includes a discussion on general edge stretch issues in forming operations, including material data that demonstrate the higher stretch limit of SFHS steel as compared to other high strength steels. In a case study, SFHS steel is applied to a control arm and finite element analysis (FEA) is conducted to evaluate forming and structural performance.

Forming Limit Diagrams (FLD) have been widely and successfully used in sheet metal stamping as a failure criterion to detect localized necking, which is the most common failure mechanism for conventional steels during forming. However, recent experience from stamping Dual-Phase steels found that, under certain circumstances such as stretching-bend over a small die radius, the sheet metal fails earlier than that predicted by the FLD based on the initiation of a localized neck. It appears that a different failure mechanism and mode are in effect, commonly referred to as “shear fracture” in the sheet metal stamping community. In this paper, experimental and numerical analysis is used to investigate the shear fracture mechanism. Numerical models are established for a stretch-bend test on DP780 steel with a wide range of bend radii for various failure modes. The occurrences of shear fracture are identified by correlating numerical simulation results with test data.

Different die surface polish conditions result in a noticeable effect on material flow in stamping, which can lead to splitting, wrinkling, or other surface stretching issues associated with different friction conditions. These occurrences are not only limited to the non-coated dies, but also nitrided and chrome plated dies. To ensure quality control of the stamped parts, the die conditions corresponding to different polishing procedures need to be developed based on measurable parameters such as surface roughness (Ra). The intent of this study is to investigate the effects of nitrided and chrome plated die surface roughness on friction. The Bending-Under-Tension (BUT) test was conducted to simulate the stamping process due to the test’s versatility and flexibility in changing test parameters. The test involves moving sheet metal across a 3/8-inch diameter pin, which substitutes for a die surface. The pin can be modified by material, heat treatment, coating, and surface roughness.

Fracturing in a tight radius during bending is one of the major manufacturing issues in forming Advanced High Strength Steels (AHSS). The study investigated the bendability of AHSS under two forming conditions: bending with and without stretched over the die radius. The bendability was evaluated by conducting modified Bending Under Tension (BUT) test for stretch bending and 90o v bend test for bending without stretch. The study also examined the effect of material properties on the limiting bend ratio. Various strength high strength steels, range from 420 MPa to 1700 MPa tensile strength, were selected in the study. Results indicated that critical radius-to-thickness ratios between the two tests are different but correlated in a relationship which was depicted in the bendability diagram.

Advanced high strength steels (AHSS) have been extensively used in the automotive industry for vehicle weight reduction. Although AHSS show better parent metal fatigue performance, the influence of material strength on spot weld fatigue is insignificant. To overcome this drawback, structural adhesive can been used along with spot weld to form weld-bond joints. These joints significantly improve spot weld fatigue performance and provide high joint stiffness enabling down-gauge of AHSS structures. However, modeling the adhesive joints using finite element methods is a challenge due to the nonlinear behavior of the material. In this study, the formulation of cohesive element based on the traction-separation constitutive law was applied to predict the initiation and propagation of the failure mode in the adhesively bonded joints for lap shear and coach peel specimens subjected to quasi-static loadings. The predicted load versus displacement relations correlated well with the test results.

As a result of the increasing usage of high strength steels in automotive body structures, a number of formability issues, particularly bend and edge stretch failures, have come to the forefront of attention of both automotive OEMs and steel makers. This investigation reviews these stamping problems and attempts to identify how certain material properties and microstructural features relate to forming behavior. Various types of dual phase steels were evaluated in terms of tensile, bending, hole expansion, limiting dome height, and impact properties. In addition, the key microstructural differences of each grade were characterized. In order to understand the material behavior under practical conditions, stamping trials were conducted using actual part shapes. It was concluded that material properties can be optimized to maximize local formability in stamping applications. The results also emphasize that the dual phase classification can encompass a broad range of property variations.

The front rail plays a very important role in vehicle crash. Trigger holes are commonly used to control frame crush mode due to their simple manufacturing process and flexibility for late changes in the product development phase. Therefore, a study, including CAE and testing, was conducted on a production front rail to understand the effects of trigger hole shape, size and orientation. The trigger hole location in the front rail also affects crash performance. Therefore, the effect of trigger hole location on front rail crash behavior was studied, and understanding these effects is the main objective of this study. A tapered front rail produced from 1.7 mm thick DP600 steel was used for the trigger hole location investigation. Front rails with different trigger spacing and sizes were tested using VIA sled test facility and the crash progress was simulated using a commercial code RADIOSS. The strain rate, welding and forming effects were incorporated in the front rail modeling.

The use of advanced high strength steels (AHSS) such as dual phase (DP), transformation induced plasticity (TRIP) and stretch flanging (SF) steels of the tensile strength of 600 MPa range are well established in automotive components production. This is due to their superior crash energy absorption ability and vehicle weight reduction potential. Recent trends show rapid growth in applications of even higher strength grades such as 800 MPa and 1000 MPa tensile strength and above. They are mostly used for fabrication of crash sensitive components to meet much higher safety requirements in side impact and roll-over accidents. One of the few concerns during the fabrication of AHSS components is the formability limit in flanging and hole expansion operations. Questions have been raised about the applicability of existing manufacturing experience with conventional high strength low alloy steels (HSLA) to new generations of AHSS.

More and more advanced high strength steels (AHSS) such as dual phase steels and TRIP steels are implemented in automotive components due to their superior crash performance and vehicle weight reduction capabilities. Recent trends show increased applications of higher strength grades such as 780/800 MPa and 980/1000 MPa tensile strength for crash sensitive components to meet more stringent safety regulations in front crash, side impact and roll-over situations. Several issues related to AHSS stamping have been raised during implementation such as springback, stretch bending fracture with a small radius to thickness ratio, edge cracking, etc. It has been shown that the failure strains in the stretch bending fracture and edge cracking can be significantly lower than the predicted forming limits, and no failure criteria are currently available to predict these failures.