Effect of susceptibility to interfacial fracture on fatigue properties of spot-welded high strength sheet steel

While advanced high strength steels (AHSS) have numerous advantages for the automotive industry, they can be susceptible to interfacial fracture when spot-welded. In this study, the susceptibility of interfacial fracture to spot-weld microstructure and hardness is examined, as well as the corresponding relationships between fatigue, overload performance, and interfacial fracture for a TRIP (transformation induced plasticity) steel. Simple post-weld heat-treatments were used to alter the weld microstructure. The effect on interfacial fracture of diluting the weld pool by welding the TRIP material to non-TRIP steel was examined, along with the effect of altering the base material microstructure. Results show that weld hardness is not a good indicator of either the susceptibility to interfacial fracture, or the strength of the joint, and that interfacial fracture does not necessarily lead to a decrease in strength compared to conventional weld-failure mechanisms, i.e. button pullout. It was also found that while interfacial fracture does affect low cycle to failure behavior, there was no effect on high cycle fatigue.

Influence of low-strain deformation characteristics of high strength sheet steel on curl and springback in bend-under-tension tests

The influence of low-strain deformation behavior on curl and springback in advanced high strength steels (AHSS) was assessed using a bend-under-tension test. The effect of yielding behavior on curl and springback was examined by heat-treating two dual-phase steels to induce yield point elongation, while keeping a relatively constant tensile strength and a constant sheet thickness. A dual-phase and TRIP steel with similar initial thickness and tensile strengths were also examined to investigate the effect of work-hardening on curl and springback. It is shown that while current understanding limits prediction of curl and springback in bending under tension using only the initial sheet thickness and tensile strength, both the yielding and work-hardening behavior can affect the results. Explanations for these effects are proposed in terms of the discontinuous yielding and flow stress in the materials.

Effects of aging bake hardenable steel in sheet metal forming

Bake hardening steels are being used as body panels in modern cars. These steels are stronger through an ageing process after baking in the paint ovens, which allows the thickness of the steel to be reduced without reducing the denting performance. The current study examined whether the steel ages at room temperature prior to forming which would deteriorate the formability. The work developed a new test and provided insight into acceptable levels of natural ageing which could be tolerated in the manufacturing process.

Vanadium microalloying for ultra-high strength steel sheet treated by hot-dip metallising

Ultra-high strength steel sheets have been subjected to heat treatments that simulate the thermal cycles in hot-dip galvanising and galvannealing processes and evaluated with respect to their resulting mechanical properties and microstructures. The steels contained suitable contents of carbon (∼0.2%), manganese (1.2%) and chromium (0.4%) to ensure that they could be fully transformed to martensite after austenitisation followed by rapid cooling in a continuous annealing line, prior to galvanising. Different contents of vanadium (0–0.1%) and nitrogen (0.002–0.012%) were used to investigate the possible role of these microalloying elements on the strength of the tempered martensite. Vanadium, especially when in combination with a raised nitrogen content, helps to resist the effect of tempering so that a larger proportion of the initial strengthening is preserved after the galvanising cycle, giving tensile strength levels exceeding 1000 MPa. Different deoxidation practices using aluminium or silicon have also been included. These showed similar strength levels at corresponding carbon contents but the bendability of the Si-killed steel sheet was considerably superior. Microstructural examinations have been made on the annealed steels but the reason for the beneficial effect of vanadium is still not fully explained. It is concluded that microalloying with vanadium is a very promising approach in the development of corrosion-resistant ultra-high strength steel sheet products.

The role of damping in steel pan manufacture

Results are presented of systematic studies of vibration damping in steel of a type, and processed by a route, rel-evant to Caribbean steel pans. Damping is likely to be a significant factor in the variation of sound quality be-tween different pans. The main stages in pan manufac-ture are simulated in a controlled manner using sheet steel, cold-rolled to a prescribed level of thickness reduc-tion then annealed at a chosen temperature in a laboratory furnace. Small test strips were cut from the resulting material, and tested in free-free beam bending to deduce the Young’s modulus and its associated loss factor. It is shown that the steel type, the degree of cold working and the annealing temperature all have a significant influence on damping. Furthermore, for each individual specimen damping is found to decrease with rising frequency, ap-proximately following a power law. Comparison with samples cut from a real pan show that there are further influences from the pan’s geometrical details.

F. E.-assisted design of the eaves bracket of a cold-formed steel portal frame

Non-linear large-displacement elasto-plastic finite element analyses are used to propose design recommendations for the eaves bracket of a cold-formed steel portal frame. Owing to the thinness of the sheet steel used for the brackets, such a structural design problem is not trivial as the brackets need to be designed against failure through buckling; without availability of the finite element method, expensive laboratory testing would therefore be required. In this paper, the finite element method is firstly used to predict the plastic moment capacity of the eaves bracket. Parametric studies are then used to propose design recommendations for the eaves bracket against two potential buckling modes of failure:

Characterisation of galvanneal coatings on strip steel

Galvanneal is a form of zinc-coated sheet steel, where steel is dipped in molten zinc, and then heat treated in a furnace to produce a complex iron-zinc coating. Many industries, such as automotive, use galvanneal for components fabricated from sheet steel. The microstructural properties of galvanneal have a significant influence on how well the sheet metal changes shape on stamping. By means of optical microscopy, scanning electron microscopy, and glow-discharge optical emission spectrometry, we present a study of the microstructure of several galvanneal samples, both stamped and unformed, relating the phases and morphology of the coatings to performance in stamping operations. Samples of galvanneal were subjected to different heat-treatment temperatures. The frequency of defects in stamped components was found to be related to the average alloy content in the coatings, which varied with furnace temperature. An increased average iron content in the coatings was related to increased powdering defects in stamping operations that use galvanneal coated sheet steel.

Interaction between the coating characteristics of galvanneal steel and forming parameters during flat die drawing

Galvanneal steel is considered to be better for automotive applications than its counterpart, galvanized steel, mainly because of its superior coating and surface properties. Galvanneal steel is produced by hot dipping sheet steel in a bath of molten zinc with small, controlled, levels of aluminium, followed by annealing which creates a Fe-Zn intermetallic layer. This intermetallic layer of the coating improves spot weldability and improves subsequent paint appearance. However, if the microstructure of the coating is not properly controlled and forming parameters are not properly selected, wear of the coating could occur during stamping. Frictional sliding of the sheet between the tool surfaces results in considerable amount of coating loss. An Interstitial Free steel with a Galvanneal coating of nominally 60g/m2 was used for the laboratory experiments. Flat Face Friction (FFF) tests were performed with different forming conditions and lubricants to simulate the frictional sliding in stamping. Glow-Discharge Optical Emission Spectrometry (DG-OES) was used to measure the change in the coating thickness during sliding. Optical microscopy was considered for imaging the surfaces as well as an optical method to compare the changes in the coating thickness during the forming. The change to the Galvanneal coating thickness was found to be a function of forming parameters.

Analysis of springback variation in sheet metal forming

This thesis investigated how the variation in inputs, such as material and processing conditions affected the shape defect phenomenon (springback) for sheet metal forming processes. Using a stochastic Finite Element modelling tool, it was found that the material type and fluctuations in material properties significantly influenced the variation in springback.

Press forming performance of galvanneal coated steel

This project quantified the press formability of galvanneal coated steel. A coil of galvanneal coated steel was produced containing various coating structures. Using different lubricants, the powdering, friction and forming properties were determined. It was found that optimum formability was obtained by effective lubrication while galvanneal coating structure controlled powdering resistance.

USS electrical steel sheets; engineering manual.

Mode of access: Internet.