Papers by Keyword: Interstitial Free Steel

Abstract: The evolution of microstructure and texture due to recrystallization was investigated in cold rolled interstitial free (IF) steel. Samples taken from the cold band of a 0.07%Ti bearing IF steel with total cold rolling reduction of 75% were investigated by using optical microscopy, X-ray diffraction and electron back-scattered diffraction etc. The aim of this study was to obtain a real picture of the formation of the recrystallization texture of IF steel, which would contribute to proper texture control for improving deep drawability. The mechanism responsible for the evolution of texture is discussed experimentally from four aspects. First of all, the microstructure of partially annealed cold rolled specimen is obtained. The recrystallized grain form earlier in more darkly etched regions from the micrographs, which belong to the ND//<111> fiber components. In addition, the fractions of the {111}<110> and {111}<112> components increase slowly during the early stage of recrystallization, because the γ-fiber recrystallized texture is growing, while at the same time the γ-fiber deformed texture is being consumed. Moreover, The texture formation has been discussed taking into consideration of the stored energy and the misorientation between the orientation of the recrystallized grain and that of the surrounding deformed matrix. The work indicates that the recrystallized grains, which migrate into the deformed grains, are mainly with the high misorientation angles. The large percentage of the recrystallized grains, whose misorientation angles with deformed grains exceed 15°, are corresponding to the {111} transformation texture. Nucleation first starts at colonies that have the highest stored energy of deformation, which has the same orientation as the deformed grains. By analysis, the formation of recrystallization texture was well explained by oriented nucleation mechanism rather than by selective growth mechanism.

Abstract: We have investigated the susceptibility to hydrogen embrittlement of interstitial-free (IF) steel with ultrafine-grained microstructure produced by accumulative roll-bonding (ARB) process. The ARB process was conducted to as-received IF steel at 773 K, and repeated to five cycles. The as-received and the ARBed IF steels were cut into tensile specimens, and then hydrogen was electrochemically charged to the specimens in a sulfuric acid solution of pH 2.5 at a cathodic current density of 50 A m-2 for several charging times. Immediately after the hydrogen-charging process, tensile test was conducted at ambient temperature and an initial strain rate of 3.3 x 10-4 s-1. Besides, state and amount of hydrogen absorbed in the specimen were determined with a thermal desorption gas spectroscopy (TDS) at a heating rate of 5.6 x 10-2 K s-1. As a result, almost no hydrogen was absorbed in the as-received IF steel charged for a long time of ca. 300 ks, and a fracture strain of the steel was independent of the charging time. On the other hand, amount of hydrogen in the 5-cycle ARBed steel increased with an increase in the charging time, and the fracture strain decreased with an increase in the charging time, indicating that the ARBed steel exhibited susceptibility to hydrogen embrittlement.

Abstract: An IF steel sample was cold rolled to a reduction of 80% and subsequently submitted to annealing treatment at 650°C, 710°C and 770°C for various durations, respectively. The grain size and evolution of partial texture of small, medium and large grains were investigated during grain growth. It was found that the growth rate and partial texture characteristic were absolutely distinct at different annealing temperature, both were temperature dependent. At same annealing temperature, partial texture for various annealing time was similar, but its intensity increases with an increment of duration. And the relation between partial texture of different grains assembly and its mean grain size can be expressed by an empirical formula from experiment data.

Abstract: Annealing-induced hardening and deformation-induced softening behavior has recently been found in nanostructured aluminum (fcc) produced by severe plastic deformation. It has also been demonstrated that annealing led to a decrease in ductility while deformation led to an increase in ductility. These mechanical responses are totally opposite to those in conventional coarse-grained samples. The present study explores the effect of post-process annealing or deformation on mechanical properties of nanostructured interstitial free (IF) steel (bcc). Accumulative roll-bonding was used to produce the nanostructured IF steel. The deformation structure was characterized by a lamellar boundary structure with a mean spacing of about 200 nm, consisting of high-angle boundaries, low-angle dislocation boundaries and dislocations in the volume between the boundaries. When the deformed sample was annealed at 400oC for 0.5 h, the yield stress and ultimate tensile strength increased and the elongation to failure decreased markedly. In contrast, when the annealed treatment was followed by a light rolling deformation of 15 % thickness reduction, the strength decreased and the elongation to failure increased. These results are consistent with those observed in the aluminum samples. Structural observations by transmission electron microscopy indicated that a removal of dislocations between the boundaries leads to a lack of dislocation sources, resulting in a higher stress to activate alternative dislocation sources. It was suggested that deformation rather than annealing could be a new route to improve the ductility of nanostructured metals and that a moderate light deformation gives a good balance of strength and ductility.

Abstract: A simple and reliable method has been proposed for determining fracture toughness of thin sheets. The principle of the method considers that critical crack opening displacement (c) corresponds to a specific amount of load drop during fracture toughness tests. The suggested technique yields c value for an interstitial free (IF) steel as 2.04 mm in excellent correspondence with an indirect estimate of 1.97 mm from the popular energy extrapolation technique. The magnitude of c for IF steel sheets is found to decrease with decreasing thickness in agreement with the expected variation of this criterion with specimen thickness in gross yielding fracture mechanics (GYFM) regime.

Abstract: The present paper reports on the effect of texture on the cyclic stress-strain response (CSSR) and the fatigue life of ultrafine-grained (UFG) interstitial-free (IF) steel. Tests in the lowcycle fatigue (LCF) regime were conducted on material that was processed by the equal channel angular extrusion (ECAE) technique along the so called “efficient” route 8E. This route has been shown to result in a homogeneous microstructure with a high fraction of high angle grain boundaries (HAGBs), which are beneficial for a stable CSSR. In addition, the evolution of the microstructure was characterized by means of electron optical techniques, including electron backscattered diffraction, and by X-Ray diffractometry. It was found that the initial texture of specimens cut from the ECAE billet along different orientations with respect to the extrusion direction (ED) has a substantial effect on the CSSR of the UFG IF steel. Furthermore, microscopy results indicated the notable influence of the last ECAE processing step on the evolution of damage in the material.

Abstract: This paper reports on the fatigue performance of an ultrafine-grained (UFG) interstitialfree (IF) steel deformed at various mean stress levels. The UFG microstructure was achieved using equal channel angular extrusion processing at room temperature (RT) and along an “efficient” route, giving way to the formation of high angle grain boundaries (HAGBs) with a high volume fraction. The current study not only confirms the previous finding that a high volume fraction of HAGBs promotes cyclic stability, but also inquires into the role of mean stress level on the cyclic stability. It is shown that the UFG IF steel exhibits a stable cyclic deformation response in the lowcycle fatigue regime within the medium applied mean stress range of -75 to 75 MPa. The corresponding fatigue lives can still be predicted with the Smith-Watson-Topper approach within this range. Furthermore, the present study demonstrates that the evolution of mean strains with cyclic deformation can be linked to the evolution of mean stresses in strain-controlled loading.

Abstract: Interstitial-free (IF) sheet steels are largely used in automotive deep-drawing applications, as they offer both good formability and adequate strength for auto bodies. In order to improve the corrosion resistance they are usually coated, mostly by hot-dip galvanizing. In the present work two IF steels purposely designed for a specific continuous hot-dip galvanizing line are investigated. The employed methodology, based on physical simulation, is presented and discussed in the paper. Its reliability is demonstrated by comparing some experimental and industrial results. The simulated specimens are tensile tested and the effects of annealing temperature and line speed on their mechanical properties are evaluated.

Abstract: Plane strain compression tests have been carried out on Ti stabilised interstitial free steel at 700oC with constant and changing strain rates. Specimens were annealed in a salt bath at 750oC to determine the effects of changing strain rate on the kinetics of static recrystallisation and on the recrystallised grain size. After relatively slow changes in rate, the recrystallisation behaviour at the end of the change was the same as for tests at constant strain rate with the final value. For faster changes in rate, there were transients in recrystallisation rate and recrystallised grain size at the end of the change in strain rate at a strain of 1.0. These were removed by a further increment of 0.1 strain at constant rate. In all cases the recrystallised grain size correlated with the subgrain size present at the end of deformation.

Abstract: Industry hot deformation processes such as hot rolling are complex in nature. Setting up a rolling mill requires precise knowledge of the loads needed to shape the metal. This in turn, demands the ability to predict the strength of the material when deformed to a value of strain and strain rate at a given temperature. On and off-line models need, however, to be fed with constitutive equations relating the stresses required to deform a certain metal under the usual process variables. This paper shows how a set of stress-strain curves can be modeled so that both hardening and softening mechanisms commonly present during hot deformation are taken into account. The model predictions are compared to a set of literature data in order to be validated. Reasonable agreement between published results and predicted values were obtained indicating how efficiently the model can assess values of stresses under hot working conditions.