Materials Science Forum Vols. 539-543

Abstract: Thermomechanical controlled processing of concrete reinforcement bars, comprising quenching and autotempering, produces a microstructural gradient across the diameter - tempered martensite near to the surface, bainite and/or degenerated pearlite in the intermediate layers and pearlite-ferrite within the core. Since martensite is the strength controlling phase in steels, its fractional thickness in TMCP bars have beeen correlated to tensile properties. The developed empirical model, helped in revealing that upto a fractional rim of about 20%, volume fraction of ferrite-pearlite predominantly influenced the YS, UTS and % elongation, whereas at higher fractions, martensite found to control these properties.

Abstract: Thin slab continuous casting (TSCC) has recently become a winning technology in production of automotive steels in particular those of deep-drawing quality for car body panels. By combining the TSCC with direct rolling while retaining the heat in the process, thin steel sheets can be produced with high efficiency at much lower cost than with the former cold rolling / annealing procedures. However, difficulties appear when higher strength medium-alloyed or micro-alloyed steels have to be manufactured by this method. Some alloying elements, which tend to segregate, make the as-solidified dendritic microstructure prone to hot cracking and then require well-controlled cross-reductions during subsequent rolling or thermal-mechanical treatment to homogenize microstructure and get optimum properties. Physical simulation appears to be an adequate method to generate dendritic microstructures characteristic of the continuously cast thin slabs, thus avoiding costly experiments on full- scale production lines. The direct rolling simulator HDS-V40, designed and manufactured by Dynamic Systems Inc., allows controlled melting and solidification of a central portion of 10*50*165mm flat bar, and after solidification deforming the dendritic microstructure by multistep plane strain compression to simulate hot rolling. In the molten pool formed in the sample, the dendrites grow similarly like it appears in the industrial thin slab casting, i.e. from the crystallization front defined by main directions of the heat flow to the supporting base and to the (already) solid portion of the slab. In the paper presented are metallographic evidences confirming the accuracy of this physical simulation method.

Abstract: The strip casting of steel, whereby liquid steel is solidified between twin water cooled copper rolls directly into its final shape, is a radical, energy efficient, cost effective route for the production of steel products that also provides exciting opportunities for the development of new products. An experimental program is currently underway to study phenomena associated with rapid solidification of steel using levitating droplet techniques and Gleeble®3500 thermo-mechanical processing. For example, studies have been conducted to investigate the heat transfer, nucleation behaviour and microstructure development during solidification of a low carbon steel and a peritectic steel on copper substrates hard coated electrolytically or using Filtered Arc Deposition (FAD). It was found that peak and average heat fluxes were significantly higher for steels solidified on the first substrate than for the FAD coated substrates. Maximum heat flux on the respective substrates was 36.5 to 39.0 MW/m2 and 8.3 to 9.4 MW/m2. The average heat flux on the respective coated substrates ranged between 9.6 to 12.5 and 5.5 to 6.6 MW/m2.

Abstract: This paper presents a physically based precipitation model which aims at describing precipitation kinetics when it occurs exclusively on dislocations. We present specific nucleation, growth and coarsening equations, which are integrated in a set of differential equations. This model is successfully applied to the case of precipitation of NbC in a ferritic steel, whose kinetics has been determined by small-angle neutron scattering.

Abstract: In order to investigate the effects of Nb carbo-nitride precipitation conditions on abnormal grain growth behavior during high temperature carburizing, size of Nb carbo-nitride precipitates was controlled by precipitation treatment at 1173-1273K for 0.6-54ks, and the specimens were quasi-carburized at 1323K. Abnormal grain growth was enhanced when the size of Nb precipitates was fine or coarse, so there is a suitable size range in Nb precipitates to suppress abnormal grain growth. The reason why abnormal grain growth was enhanced is the lack of pinning force as the conventional theory proposed by Hillert or Gladman; however, it cannot be explained by this theory that small precipitates promote abnormal grain growth. It is considered that Ostwald ripening rate of precipitates is also an important factor in controlling abnormal grain growth in addition to the amount and size of precipitates and austenite grain size, which were parameters in the Gladman‘s theory on abnormal grain growth behavior.

Abstract: In extra-low carbon steels, the generation of specific nucleation sites in the deformed microstructure determines the formation of an optimum recrystallization texture. In particular, during the cold rolling of the steel sheets, transition bands are generated in order to accommodate the different deformation paths followed by the deformation bands (DB-s) within the grains. -fibre grains (ND-fibre grains) are, in general terms, more fragmented than -fibre grains (RD-fibre grains). Consequently, the higher orientation gradients and stored energy levels of the -fibre grains determine the ND-fibre annealing texture. Nevertheless, during recrystallization of the ELC steel, nucleation in different type of transition bands (TB-s) between α-fibre components has also been observed in the present work. From a previous crystallographic classification of transition bands done by the authors, the effect of the lattice curvature, either by gradual or sharp orientation gradients, is studied during early recrystallisation stages. Specifically, the crystallographic characteristics of recrystallized nuclei formed at transition bands generated between α-fibre components are analyzed using electron back-scatter diffraction (EBSD). This technique enables the orientation of deformation bands, the misorientation across them, the orientation of the new recrystallized grains and the misorientation of those grains with the adjacent matrix grains to be determined. Recrystallization of components different to α at the expense of α components has also been found.

Abstract: The nitriding process is one of the common methods for surface hardening, and consists of heat treatment in a furnace for many hours. The nitriding behavior and strengthening mechanism of Ti added steels in the nitriding process, which is applicable to a high temperature and rapid process such as the continuous annealing of steel strip, were investigated. The sheets were hardened only near the surface. The hardening of the surface layer is due to the formation of clusters or fine precipitates with disc-like shape consisting of titanium and nitrogen. The maximum hardness depends on the content of Ti in the steel while the annealing time and the concentration of NH3 influence the depth of the hardened zone affected by nitriding. It is thought that the hardening only near surface improve bending stiffness without significant increase of yield stress. So there is a possibility that the surface hardening improves the dent resistance of automobile outer panels without significant worsening of the surface deflection. To study these behaviors theoretically, a model for predicting the precipitation behavior due to nitriding has been developed. The experimental results can be reasonably explained by the model calculations. And also, the estimation of the amount of strengthening was carried out. It indicated that the strengthening mechanism is mainly the precipitation hardening of TiN that could be Ti nitrides or Ti-N clusters.