Materials Science Forum Vols. 638-642

Abstract: Thermodynamic data for the substitution of silicon and manganese in cementite have been estimated using first-principles methods in order to aid the design of steels where it is necessary to control the precipitation of this phase. The need for the calculations arises from the fact that for silicon the data cannot be measured experimentally; manganese is included in the analysis to allow a comparison with its known behaviour. The calculations for Fe3C, (Fe11Si4c)C4, (Fe11Si8d)C4, (Fe11Mn4c)C4 and (Fe11Mn8d)C4 are based on the total energy all-electron full-potential linearized augmented plane-wave method within the generalized gradient approximation to density functional theory. The output includes the ground state lattice constants, atomic positions and bulk moduli. It is found that (Fe11Si4c)C4 and (Fe11Si8d)C4 have about 52 and 37 kJ greater formation energy when compared with a mole of unit cells of pure cementite, whereas the corresponding energy for (Fe11Mn4c)C4 and (Fe11Mn8d)C4 is less by about 5 kJ mol1. These results for manganese match closely with published trends and data; a similar comparison is not possible for silicon but we correctly predict that the solubility in cementite should be minimal.

Abstract: The secant method proposed by Weng [1] is a practical calculation method to evaluate the stress-strain curve of the two-phase materials, but the shape of the inclusion phase has been often assumed to be a sphere or an ellipsoid in the calculation. In this study, we modified the secant method by utilizing the phase-field micro-elasticity theory [2,3] so as to be able to calculate the SS-curve of the materials consisting arbitrary morphology of microstructure, and applied this method to the conventional microstructures in steels, i.e., the ferrite-bainite two-phase microstructure.

Abstract: It is very important to understand interstitial carbon behaviors in cold rolled steel to get the good formability as well as the high strength. In low carbon steel, most of carbons are consumed by the formation of grain boundary cementite during cooling. During heating and holding between Ae1 and Ae3, cementite is dissolved and consequently carbon enriched austenite is formed. By controlled cooling, retained austenite as well as bainite and martensite are formed. In this study, the effect of silicon, intercritical annealing, isothermal bainite transformation on the formation of ferritic bainite, cementite and retained austenite are modeled by nucleation and growth, diffusion and dissolution. In addition, the formation of retained austenite and their carbon contents are modeled and compared with experimental data.

Abstract: The microstructural failure mechanisms of two DP steel sheets cooled with different cooling rates during their heat treatment are compared in the present study. The as-cold rolled DP steel sheets were annealed at intercritical temperature and cooled down with rates of 45°C/s (quenching) and 2 °C/s (slow cooling). Uniaxial tensile tests were carried out on samples from both sheets and the microstructure of undeformed samples and the broken tensile specimens was evaluated by optical microscopy, scanning electron microscopy and electron back-scatter diffraction technique. Although the grain size did not show significant differences, the amount and size of the constituents, e.g. martensite and bainite, differ between both alloys. Concerning the mechanical properties, the quenched material showed superior strength and ductility besides a less localized deformation at higher strains. The area fraction of voids in the broken specimens was low for both steels. In the slow cooled samples the nucleation of shear bands was on the large voids and cracks were observed along these shears bands. It was concluded that the detrimental effect of void nucleation on both steels is not only attributed to their null-carry capacity but more to the stress concentration close to the voids which gives rise to strain localization in the form of shear bands.

Abstract: As already intensive studies related to the processing of hot rolled dual phase steels have been reported in the past, the aim of this particular paper is to present some details within a processing strategy, that can be considered as useful for the processing of other special steel types. It will be shown that based on fundamental dilatometric measurements performed in the laboratory, a narrow window of processing parameters can be focused at. Moreover, practical dilatometric curves are shown, taking into account the transformation heat release in the arrested cooling zone (dry section), as to be considered regarding the industrial practice. Furthermore, by introducing a slab insert technique within the development schedule, not only time consuming research can be limited, especially main objectives can be achieved. Some of these objectives are: a) a simultaneous study of the influences of the chemical composition of the steels, b) a close control of the thermomechanical parameters encountered in the industrial environment, such as rolling reductions, strain rate and inter pass time conditions, c) a check of the homogeneity of mechanical properties versus the coil length.

Abstract: The effect of coiling temperature on the microstructure and mechanical properties of a Nb-V microalloyed steel was investigated. Controlled rolling followed by accelerated cooling and coiling was simulated by means of both, uniaxial compression, using a quenching and deformation dilatometer, and torsion. Specimens were reheated at 1250º for 5-10 min, then deformed at 1150°C, =0.3, and subsequently at 900°C, =0.4, followed by rapid cooling to a temperature between 450 and 700°C where coiling was simulated by holding the specimen for one hour at the selected temperature followed by a slow cooling to room temperature. Mechanical characterization was performed by means of hardness measurements and tensile tests, using tubular specimens machined from the torsion samples. It was found that decreasing the coiling temperature the ferrite-pearlite microstructure changed to ferrite-bainite, with a hardness peak reached for coiling at 600°C-650°C.

Abstract: Carbides in HSS roll were studied systematically. The results showed that there were two kinds of carbides in HSS roll, 1st and 2nd carbides. 1st eutectic carbides included the chrysanthemum-like MC mainly consisting of V and the plate-like M2C rich in Mo. 2nd carbides were complex carbides, concentrating Fe, Cr, Mo and V elements. The morphology, distribution and alloy concentration of the 1st eutectic carbides were studied using OM, SEM and EDS. During heat treatment, evolution of the 1st eutectic carbides was analyzed. It was found that the MC carbides had no change in morphology, components and distribution, while the M2C carbides decomposed to M6C and MC. In order to improve the properties of HSS, a method of increasing solidification rate was defined to refine the eutectic carbides. After refinement, the eutectic carbides distributed both along the grain boundaries and inside the grains.

Abstract: The ductility behaviour experienced by steels for linepipe (LP) applications in a temperature range generally from 700°C to 1200°C is a widely studied subject in steel research, especially for its implication on cracking during continuous casting and rolling. Hot tensile / torsion tests on as-cast products, conducted until fracture, are normally used to characterise the hot deformability behaviour. Depending on the industrial hot deformation process within which the steel aptitude is being investigated, other types of tests can be more adequate. With the aim to characterize the hot deformability behaviour in terms of the damages each steel presents at moderate strain levels (i.e. far from the onset of necking), a special device for interrupted hot tensile tests, followed by immediate quenching (i.e. to “freeze” the microstructure) was developed. Various industrial steels with different starting microstructures (ad hoc in-lab heat treatments performed before testing) were tested by this method, and subsequent metallurgical investigations of the strained samples were carried out to identify, for each case, the damage mechanism and the microstructure features having the major influence on ductility loss. As a result, it was found that (i) microstructural damages at moderate strain levels can be much better described throughout interrupted hot tensile tests, (ii) different compositions and starting microstructures within the industrial LP scenario lead always to microstructural damages at relatively high deformation temperatures (e.g. 950°C) and moderate strains (e.g. 0.1 to 0.2, very far from the onset of necking), (iii) the common mechanism by which LP steels start voiding is the grain boundary sliding and (iv) the intergranular voids, once formed, grow longer in coarser microstructures.

Abstract: Ultra rapid annealing cycles were conducted on two low carbon Al-killed steel sheets differing mainly by their coiling temperatures (600°C or 700°C). For the lowest coiling temperature, the mean grain size of the steel was found to gradually decrease with an increase of the annealing temperature from 700°C to 920°C. A more complex grain size evolution was detected in the case of the steel coiled at high temperature. This led us to the conclusion that the size and the distribution of the iron carbides present before annealing, which is mainly governed by the coiling temperature, plays a very important role on the mechanisms involved in the grain refinement of extra-mild steels during ultra-rapid annealing cycles.

Abstract: Industrial low alloy TRIP sheet steels contain blocky and lath-shaped retained austenite. In the present study, transformation behaviour of blocky and lath-shaped retained austenite during straining was investigated to clarify its effect on mechanical properties. Two types of TRIP steels containing almost the same amount but the different morphology of retained austenite were used. A steel containing large amount of lath-shaped retained austenite exhibits superior ductility, and sustains high work-hardenability in a high strain region. On the contrast, a steel containing large amount of blocky retained austenite exhibits low ductility. 　The work-hardenability increased steeply to the maximum at a low strain region, and then reduced in a high strain region. The stability of the blocky austenite has been found to be poor with respected to martensite transformation. The lath-shaped retained austenite remains until a high strain region whereas the blocky retained austenite transformed into martensite in a low strain region. Carbon content was higher in the lath-shaped retained austenite than in the blocky retained austenite. Stability of retained austenite is, however, inexplicable only by the carbon content, and would be affected by the different morphology and the resulting restraint conditions.