Materials Science Forum Vols. 706-709

Abstract: In this work the formation of microstructural banding in a dual-phase steel is investigated by using a three-dimensional cellular automata model for phase transformations. Originally developed for describing the metallurgical processes occurring during the annealing stage of cold-rolled strips, this model is presently applied to investigate microstructural-band formation during the austenite-to-ferrite transformation kinetics during cooling after hot rolling. The recent incorporation in the model of an inhomogeneous concentration of Mn, the alloying element most responsible for the development of microstructural banding, and the local nucleation behaviour dependent on the Mn concentration allows the study of the effect of material and process parameters on the banding formation.

Abstract: In the present study, the effects of kinematic and geometric asymmetries in rolling during multi-pass processing of IF steel are examined. The theoretical investigation by final element simulations and experimental investigations by means of electron-backscatter diffraction analysis and tensile tests suggest that asymmetric rolling increases the total imposed strain compared to symmetric rolling, and largely re-distributes the strain components due to additional shear. This enhances the intensity of grain refinement, strengthens and tilts crystallographic orientations, and increases mechanical strength. The effect is highest in the asymmetric rolling with differential roll diameters.

Abstract: Hot deformation of a continuously cast low alloyed steel is studied by means of hot compression and tensile tests carried out after austenitization between 700–790 °C at 3x10^-4 – 0.3 s^-1 of strain rate. The ferrite transformation at the applied cooling rate was determined at 710°C by means of dilatometry. The compressive flow data obtained by using a Gleeble®1500 machine are evaluated to obtain the strain rate sensitivity and the processing maps using different models. The tensile data are used to determine the ductility of the material with different deformation parameters. A new calculation method is used for the instability parameter derived from the dynamic materials model. The strain rate sensitivity does not predict any instability but all the others instability parameters do, including the new one. Pores are formed at the prior austenitic grain boundaries at low strain rates, causing a decay of ductility in the tensile samples. A minimum in the ductility was observed for low strain rates at 750°C. Low strain rates and low temperatures increase the formation of more ferrite than without deformation at the corresponding heat treatments without deformation. In these conditions, the deformation is concentrated in the softer ferrite phase. Low power efficiency was calculated at high strain rates, where no dynamic recrystallization takes place. The domains with similar efficiency of power dissipation are correlated to deformation induced ferrite formation and ferrite recovery. These domains vary with the increasing strain.

Abstract: The effects of annealing prior to cold rolling on the microstructure and magnetic properties of a low-C grain non-oriented (GNO) electrical steel strip have been investigated. It is shown that annealing of the hot-rolled strips in the intercritical region, Ac₁3, causes rapid decarburization and development of large columnar ferrite grains. This microstructure leads, after cold-rolling and a fast annealing treatment at temperatures between 800 and 850 °C, to a polygonal ferrite grain microstructure with magnetic properties superior to those observed typically in the same steel in the industrial fully processed condition. The results are attributed to the {100}-fiber texture developed during the final annealing. Annealing at T<800 °C or T>850 °C results in formation of {111}-fiber texture components due to recristallization or transformation of deformed ferrite leading to a negative effect on the final magnetic properties. The results suggest that annealing prior to cold rolling offers an attractive alternative processing route for the manufacture of fully processed low-C, Si-Al GNO electrical steels strips.

Abstract: In this paper the recrystallisation kinetics of cold rolled strips as a function of magnetic field intensity has been studied on a Fe-0.05C-1.5Mn alloy. The retarding effect on recrystallisation due to the application of a magnetic field on samples treated with magnetic field up to 10T, has been experimentally confirmed. The effect was more evident for high magnetic field and for long annealing times. In addition, a small retarding effect of magnetic field on grain growth has been noticed.

Abstract: The developments of plates are strongly driven by demands from high-sophisticated pipeline and structural applications. Such needs reflect in plates with higher thickness, larger width, higher toughness also at lower temperatures, higher strength and/or better resistance to sour environments. The requirement profiles are, in some cases, exceptional and have become achievable, in an extreme combination of alloying and processing technologies, only as a result of comprehensive development work and plant investments. An outline of modern production facilities is given. A broad potential for innovation derives by the utilization of the key-technology TMCP (Thermo-Mechanical Controlled Process). As a central element to achieve high-class requirements, aspects and parameters of the metallurgical design are explained. The utilization is explained and illustrated with selected recent applications.

Abstract: Improving the steel properties and production processes with reduced energy demand for high strength steels requires improved process control in close relation to the steel composition. Hot rolling of steel is an energy-intensive process, especially in respect of preheating the steel slabs. The present work was carried out with the aim of reducing the initial slab temperature while at the same time improving properties by optimization of the steel composition and process parameters. Optimization of slab reheating and hot rolling parameters in connection with plate and strip rolling was carried out on low C-Mn high-strength steels microalloyed with Mo-Ti-Nb-B, both in laboratory and full scales processing. The effects of a low slab reheating temperature, high finish rolling temperature (FRT) during thermo-mechanical controlled processing (TMCP) and accelerated cooling rates following hot rolling to RT or to the coiling temperature have been investigated. Improvement of yield strength of the plate has been obtained by lowering the slab reheating temperature, especially with high cooling rates (>20°C/s) to room temperature. The results obtained for strip steels also show that a reduced reheating temperature combined with high finish rolling temperatures and cooling rates (>20°C/s) to a coiling temperature of 450°C produces very positive microstructures and mechanical properties in the present steels. Lowering the slab reheat temperature reduces energy consumption and accordingly releases less CO₂ into the atmosphere during the thermo-mechanical processing of the present steels.

Abstract: An experimental steel containing in weight % 0.2C-2.0Mn-1.5Si-0.6Cr has been laboratory hot rolled, direct quenched into the M_s - M_f range and partitioning annealed in order to explore new possibilities for making 1100 MPa yield strength structural steels with better combinations of strength, ductility and impact toughness. Two austenite states prior to quenching were investigated: recrystallized and strained. Gleeble simulations were used to determine appropriate cooling rates and cooling stop temperatures for obtaining martensite fractions in the range 70-90%. Desired martensite - austenite microstructures were achieved, and ductility and impact toughness were better than those obtained via the simple direct quenching of a lower carbon steel in the same strength class.

Abstract: The recent major achievements obtained during the development of advanced high strength steels (AHSS) at Nucor Corporation are briefly described in this article. Due to the innovative chemistry designs as well as strictly controlled thermomechanical schedules at CSP^® and down-stream processing, these newly developed AHSS products possess fine, uniform microstructure, and exhibit higher strength-elongation balance, better formability and stretch flangeability, improved impact toughness and crashworthiness, as well as superior weldability and weld fatigue properties. Moreover, the property consistency of these innovative AHSS has been markedly improved.

Abstract: An experimental study (physical modeling) of the processes of austenite microstructure evolution occurring under hot rolling was performed for line-pipe steels with different chemical composition. All investigations were conducted with the help of the Gleeble 3800 system. Empirical quantitative models of austenite grain growth, static and dynamic recrystallization, as well as a flow stress model were developed. The effect of complex alloying by such elements as C; Mn; Si; Ni; Mo; Nb; Ti; and V on grain growth and recrystallization is accounted for under the condition that all elements are in a solid solution. The set of the models empirical parameters is determined utilizing corresponding experimental data available from literature. Modeling results for static recrystallization and flow stress in the investigated steels are compared with experimental data.