Deep Drawing Quality Steel Research Articles

Some aspects on fracture limit diagram developed for different steel sheets

In this work, forming limit diagrams with fracture limit have been evaluated for various steel sheets namely interstitial free (IF) steel with thickness 0.6, 0.85, 0.9, 1.2 and 1.6 mm, 0.85 mm noncoated and 0.85 mm coated, high strength low alloy (HSLA) steel with thickness 1.6 mm, extra deep drawing quality (EDDQ) steel with thickness of 1.2 mm and stainless steel 430 grade of 1.2 mm. The fractography of the sheets were taken using scanning electron microscope for the above said steel sheets under various conditions namely tension–tension strain condition, plane strain condition and tension–compression strain condition. Using the fractography, the average size of voids developed at fracture under the above said conditions were measured. The average size of voids for various sheets mentioned above were correlated with various shear strains, the ratio of mean strain to effective strain and other factors involving various shear strains, strain hardening exponent and normal anisotropy values. The ratio of height of the dome to punch radius ( H/ R) for various sheets mentioned above was correlated with the mean strain and also with the ratio of mean strain to effective strain. The average void sizes were also correlated with the ratio of Mohr's circle shear strain ɛ 31 to effective strain.

Numerical and experimental investigations of forming limit diagrams in metal sheets

A strain forming limit criterion is widely used throughout the sheet metal forming industry to gauge the stability of the deformed material with respect to the development of a localized neck prior to fracture. This criterion is strictly valid only when the strain path is linear throughout the deformation process. There is significant data that shows a strong and complex dependence of the limit criterion on the strain path. But unfortunately, the strain path is never linear in secondary forming processes. Furthermore, the path is often to be nonlinear in localized critical areas in the first draw die. Therefore, the conventional practice of using a path independent strain forming diagram criterion often leads to erroneous assessments of forming severity process. In this study, the numerical results are based on MARC K7.1-3D finite element analysis (FEA) software using rigid-plastic flow method. This general purpose of FEA code is using both the implicit and explicit procedure. The model is employed to predict the stress–strain forming limit diagrams and thickness distribution of a deep drawing quality steel (DDQS) and aluminum alloy (AL) sheet. The predicted results agree well with those determined experimentally.

Optimization of Annealing Parameters for Improvement in Formability of Extra Deep Drawing Quality Steel

A number of annealing cycles were investigated in an attempt to find the optimum cycle that results in an attractive combination of mechanical and formability properties of an extra deep drawing (EDD) quality steel. It was found that the cycle that involved an intermediate anneal at 600 °C followed by further soaking at 700 °C resulted in the best combination of mechanical and formability properties. It was also found that the rate of heating up to 600 °C can be kept at 50 °C/h while the heating has to be done at a rate of 30 °C/h from 600 °C to the final annealing temperature of 700 °C. The desirable combination of mechanical and formability properties has been correlated with the microstructure that shows pancaking of the annealed grains accompanied by precipitation of carbides. Precipitates of carbides are more in number and smaller in size in the case of samples annealed by the cycle mentioned above compared to the ones annealed by other cycles. They are spherical in shape, which is desirable for forming applications.

Hot-Rolling Experiments on Deformation Behaviour of Oxide Scale

A new test technique developed by the authors allows to investigate the brittle-ductile behaviour of oxide scale during the hot-rolling process. Sandwich specimens were pre-oxidised and then welded so as to be gas-tight. A micro-alloyed deep-drawing quality steel (interstitial free steel) and a fine-grained low-pearlite structural steel served as test material. The experiments were performed at temperatures up to 1050 °C. Optical metallography was used to describe the changes of the scale layers. The method is quite good to describe the results qualitatively whereas it is rather difficult to derive quantitative results.

Influence of Sulfur on Formability of Galvannealed Ultra Deep Drawing Quality Steel

Influence of Sulfur on Formability of Galvannealed Ultra Deep Drawing Quality Steel

Development of Grain Interior Strain Localizations during Plane Strain Deformation of a Deep Drawing Quality Sheet Steel.

Development of dislocation substructures was characterized in an aluminum killed deep drawing quality steel at four different plane strain deformations. At and above 20% reduction, the most significant substructural feature was micro bands (MBs). MBs appeared as paired dislocation walls of 0.2–0.4μm thickness and were always at an angle of approximately 37° with rolling direction (RD). As the traces of the MBs were more than 5° of {110} and {112}, closed packed planes of the bcc system, —they were termed as first generation1) or non-crystallographic using the convention16) commonly used in fcc metals. Other than the pre-deformation high angle boundaries, MBs were the only feature with large enough misorientations necessary for optical visibility. At least for the range of strain and strain path used in the present study, the first generation MBs can be considered as the so-called grain interior strain localizations. Relative presence and effectiveness of MBs were quantified in different microtexture components from the MB spacings along TD (λ) and the average misorientation across MBs (θMB) and these appear to determine the stored energies of different microtexture components.

Influence of inclusion characteristics on the formability and toughness properties of a hot-rolled deep-drawing quality steel

In industrial practice, variations in the steelmaking process may cause significant change in inclusion characteristics. During hot rolling of flat steel products, manganese sulfides, which are plastic at elevated temperatures, are elongated in the rolling direction. These elongated inclusions affect the formability properties, such as ductility, strain hardening exponent, average plastic strain ratio, critical strain represented by the forming limit diagram, and Charpy V- notch (CVN) impact energy as well as fracture behavior. The inclusion characteristics and microstructural features of three commercially produced hot- rolled deep- drawing quality steels were evaluated and their effects on formability and impact properties were investigated. All three heats were made in a basic oxygen furnace. Two heats were teemed into ingots while the other heat was argon purged and continuous cast. These heats were then processed into 3.10 mm thick strips with identical processing parameters. Manganese sulfide stringers were found to reduce the transverse ductility, whereas yield and tensile strengths remained virtually the same in all directions. The formability parameters were not significantly affected by small variations in inclusion characteristics. However, CVN impact energy and impact transition temperature data were observed to improve with steel cleanliness. The sulfide stringers were also found to adversely affect the impact energy, transition temperature, and fracture behavior in the transverse direction.

On the Formability of Aluminum-Coated Steel Sheet

Mild steel sheet coated with aluminum was characterized in terms of mechanical behavior and formability. The layer of aluminum, 7μm - 10μm thick, was deposited by a relatively recently developed version of physical vapor deposition (PVD). The uniqueness of this particular deposition process comes from the use of a steered arc. The substrate chosen for the study was deep-drawing quality steel of the type used in the automotive industry. Tension tests provided information on ductility. strain, and strength. A forming limit diagram (FLD) was constructed and angular stretch bend tests carried out to assess the formability of coated sheet. This study found a 40% increase in the yield stress of coated material over uncoated material. Additionally. coated material has lower formability than uncoated sheet. The majority of the properties of the coated sheet are attributable to strain aging of the steel (diffusion of carbon atoms to dislocations) brought about by the coating process.

Effect of stress components on fatigue crack growth rate under multiaxial stress condition: Yokobori, T.A., Isogai, T., Yokobori, T. and Koizumi, Y. J. Soc. Mater. Sci., Japan Aug 1990 39, 1106–1112 (in Japanese)

The anisotropic hardening behaviors of dual phase (DP780) and extra deep drawing quality (EDDQ) steel sheets under non-proportional strain paths were investigated. Two-step uniaxial tension tests, which consisted of the first loading in the rolling (RD) or transverse (TD) and the second loading in every 15° from the first loading axis, were conducted. For DP780 steel, a significant Bauschinger effect accompanied by a transient hardening behavior after reverse loading was the prominent phenomenon. In contrast, EDDQ exhibited stress overshooting followed by strain hardening stagnation with respect to the monotonic flow curve near cross-loading conditions. The extended HAH model combined with the Yld2000-2d yield function were used to reproduced the anisotropic hardening behavior of the two materials. For DP780, the extended HAH model could capture the Bauschinger effect and transient hardening behavior well for tension reloading at 0°, 15°, 75° and 90° from the RD or TD prestraining direction. However, the predictions at 30°, 45° and 60° were slightly different from the experiments. For EDDQ, this approach reproduced the strain hardening anisotropy well including flow stress overshooting followed by a stage of strain hardening stagnation.

Numerical and Experimental Investigations of Deep Drawing of Metal Sheets

Results of a comprehensive study on deep drawing are presented. The numerical results are based on finite element procedure developed for simulation of arbitrarily shaped, 3-D forming operations. The elastic-plastic material description with Hill’s anisotropic model is employed allowing for elastic effects, unloading, and multistage processes. By applying a proper computational model of large strain and large rotation plasticity, reliable results were obtained even using relatively simple material and friction laws. Predicted values of the press force, strain distributions, and flange reduction are compared with the corresponding results from axisymmetric deep draw experiments for a wide range of materials used in real forming applications including a deep drawing quality steel, brass, high strength steel, stainless steel, and aluminum. The parametric study shows the sensitivity of the solution on material parameter variations. Presented results can be considered as a set of benchmark problems.

Surface Characteristics of Continuously Annealed Steel Sheet in Overaged Condition

The overaging treatment of a continuous annealing line was simulated in the laboratory. The chemical state of the surface of low carbon steel sheet was determined by using ESCA and SEM. The surfaces of a conventional, low alloyed deep drawing quality (DDQ) steel and a dual phase (DP) steel alloyed with Mn and Si were characterised in simulated and production line conditions. The influence of the surface state of the steels on the formation of surface phosphates was investigated. During overaging in the temperature range 200–400° C in a protective H2 + N2 atmosphere iron oxide is reduced to metallic iron. The alloying elements manganese and silicon are oxidised. Silicon especially is markedly enriched on the surface, in the form of pure Si02, due to its high diffusibility. The manganese and silicon oxides form particles on the surface during the overaging treatment. The results indicate that iron oxide is dissolved in the particles. Sodium and calcium are also present in their oxide states on the surfaces of the production line sheets; they originate in the rinsing waters used. Temper rolling reduces the amount of surface oxide particles to about half the prerolling value. It is suggested that this is due to the stripping of particles from the surface and the pressing of particles into the steel matrix. The phosphating experiments show that the steel sheets treated in the simulation furnace and taken from the production line form surface phosphates readily. The phosphate crystals are small, uniform, and cover the entire surface.

Influence of prestraining and ageing on fatigue properties of high-strength sheet steels

Strain-controlled fatigue tests and tensile tests were performed on four sheet steels in cold-rolled gauges. The fatigue tests involved fully reversed straining cycles. Two dual-phase steels with tensile strengths of 400 (DP400) and 600 MPa (DP600), one HSLA steel (HSLA500) with a tensile strength of 500 MPa and a deep-drawing quality steel (DDQ) were incorporated. The materials were exposed to different pretreatments before testing, such as prestraining in uniaxial tension or equibiaxial stretching or an ageing treatment. The materials often showed cyclic softening during the fatigue tests in as-received condition or after pretreatments. The cyclic stress amplitude at half fatigue life increased in the same order as the tensile strengths of the as-received materials, ie from DDQ, DP400, HSLA500 to DP600. Both prestraining and ageing caused significant increases of the monotonic flow stress. The influence on the cylic stress amplitude was however much smaller. The ranking of the steels with respect to fatigue life was sensitive to the choice of characteristic parameter. When life was plotted as a function of stress amplitude the life fell in the order DP600, HSLA500 and DDQ. The same order was obtained when life was considered as a function of the Neuber factor. However, when life was evaluated vs plastic strain amplitude, the life fell in the order DDQ, DP400, DP600 and HSLA500. Fatigue life was not very sensitive to prestraining and ageing. Both small positive and negative effects on fatigue life could be obtained. A linear relation was found between the fatigue strength at fixed life and the ultimate tensile strength.

Surface Characteristics of Continuously Annealed Steel Sheet in Heat Treated and Pickled Conditions

The aim of the present work was to simulate the chemical state of the surface of cold rolled steel sheet which results from some of the treatments used on a continuous annealing line (CAL, developed by Nippon Kokan). By using ESCA, GDOS, and SEM the surfaces of two different steels were characterised. One steel was a low alloyed, conventional deep drawing quality (DDQ) steel, the other a dual phase (DP) steel alloyed with manganese and silicon. The surface was studied after four different subprocesses: as received after cold rolling; after annealing and gas cooling; after water quenching; and after pickling. On annealing at high temperature in a protective atmosphere consisting of nitrogen and hydrogen the iron oxides are completely reduced. However, the elements manganese and silicon, which have high affinities to oxygen, will form oxides on the surface, distributed as discrete particles. The DP steel, which has a relatively high bulk concentration of manganese and silicon, exhibits a larger surface coverage of these oxides than the low alloyed DDQ steel. On water quenching the metallic part of the surface, consisting of iron, is reoxidised, forming a rather thick iron oxide film that almost covers the oxide islands. This thick film is unacceptable and must be removed by pickling. Simulation of the pickling process shows that the iron oxide formed during water quenching is completely removed. The surface after pickling consists of a thin iron oxide film formed during pickling and rinsing. The manganese and silicon oxide islands are not completely removed by pickling, as can be seen from the ESCA analyses and SEM. However, pickling reduces the difference in surface coverage of these oxides between the two types of steel.

On the strain distribution during the stretch-forming of low- and high-strength sheet steels

Strain distributions during stretch-forming with a hemispherical punch have been studied for six steels in cold-rolled gauges: a deep-drawing quality steel, a re-phosphorized steel, three dual-phase steels and a HSLA steel. Strain distributions are presented for radial strain, tangential strain and thickness strain. A number of different characteristics of the strain distributions have been evaluated such as the pole-to-peak distance, the width of the strain distributions and the peak- and polar-strains. Significant differences have been observed between, on one hand, the deep-drawing quality steel and, on the other hand, the dual-phase steels. The deep-drawing quality steel gives widely separated strain peaks and thus wide strain distributions, with the state of strain at the peaks being close to plane strain, whilst the dual-phase steels have rather closely situated strain peaks and thus narrow strain distributions, with the state of strain at the peaks being close to equi-biaxial straining. The influence of material parameters such as the work-hardening exponent n and the plastic anisotropy index r on the different characteristics of the strain distribution has been explored. Differences in punch heights to failure between the six materials have been investigated: it has been found that the dual-phase steels have a favourable combination of punch height at failure and tensile strength. Previously it had been thought that this is due to there being wide smooth strain distributions for dual-phase steels but the dual-phase steels do not, however, produce wider strain distributions than, for example, HSLA steels of comparable strength. Instead, the dual-phase steels have the ability to generate a state of strain close to equi-biaxial straining over a large proportion of the cup which, combined with a high forming-limit diagram close to equi-biaxial straining, produces a large punch height at failure.

Modification of a surface profile's horizontal spacing during strip drawing

Stylus profilometry is used to quantify surface damage resulting from single-pass strip drawing (SD) of different deep-drawing quality steel sheets. Simple straightforward methods are presented to determine and follow the penetration depth, real bearing area A r , average summit density and average summit length as a function of normal load F N . By comparing the real horizontal spacing during SD with the corresponding parameters resulting from a fictitious levelling down, based on purely topological aspects of the virgin sheets prior to SD, the plastic deformation resulting from the ironing action of the dies is quantified. The variation in A r as a function of F N in SD turns out to depend mainly on the topology of the virgin sheets. The corresponding variation in the average summit length clearly shows an important contribution due to plastic deformation of the surface profile.

Explosive Forming of Low Carbon Steel Sheet into a Stepped Disc Shape

This paper deals with the explosive forming of deep drawing quality steel into a two stepped disc type shape. An attempt has been made to predict the forming parameters from theoretical considerations by equating the disc shape with an equivalent dome. Results of forming this shape in a single stage vis-a-vis forming in two stages are compared.

Formability of aluminium and steel sheets

The formability of soft aluminium, half-hard aluminium, and deep-drawing quality steel sheets was compared using the material parameters r, n, and m, and the forming-limit diagrams. Strain hardening in uniaxial and biaxial tension was found to be the the same, justifying the use of the uniaxial n valuefor the assessment of formability. The necking-limit strains of the soft aluminium and steel were quite similar, the inferior press behaviour of aluminium being a result of smaller r and m values