Papers by Keyword: Transformation

Abstract: Low alloy transformation induced plasticity (TRIP) steels have a complex microstructure consisting of ferrite, bainite and retained austenite. Their excellent mechanical properties are ascribed to the martensitic transformation of retained austenite during plastic deformation. In the present contribution, the crystallographic texture of fcc and bcc phases in TRIP steels was measured by means of orientation mapping. The austenite texture was close to a typical rolling texture of fcc metals. For bcc phase, the effects of orientation and grain size on the distribution of pattern quality were investigated. The texture of transformation product phase was separated by grain size. The transformation texture showed stronger α fiber including {113}<110> component than the recrystallization texture. It showed a good agreement with a transformation texture predicted by Kurdjmov-Sachs (KS) relationship without any variant selection.

Abstract: The competitive behaviors between recrystallization and transformation during annealing in dual phase high-strength steels are studied both by experiments and by computer simulations. The Monte Carlo code was applied to simulate such competing behavior, explained the experimental results, as follows. (1) The progress of the transformation is affected by the starting point of the transformation in the course of recrystallization. Namely, if the transformation starts at the later stage of recrystallization, the n-value in JMAK equation is relatively high. (2) If the stored energy (driving force) of the transformation is above 5 times larger than that of the recrystallization, the effect of the stored energy is quite small. (3) If the number of nuclei were increased, the transformation proceeds a little faster, and then the transformed microstructures are significantly refined. When the transformation initiates at the early stage of the recrystallization, the number of nuclei can be increased by nucleation from the deformed matrix compared with the one from fully recrystallized matrix.

Abstract: The present paper presents an overview of present and future tools which can be used by the steel manufacturer in order to control the texture of the finished sheet product. The major solidstate- transformation processes (phase transformation, plastic deformation and recrystallisation) playing a role during thermo-mechanical processing will be addressed. The physical mechanisms that give rise to the appearance of specific texture components will be discussed in detail. In addition to current state-of-the-art process technology the potential of innovative processes will be described such as accumulative roll bonding (ARB). The present paper will also pay attention to the particular role of surface textures as an additional degree of freedom allowing to control the sheet texture with the potential to enhance the {001} or {110} fibre textures for magnetic applications.

Abstract: The effect of niobium (Nb) addition on recrystallization texture formation in cold-rolled low carbon steel sheets containing 2% manganese (Mn) was investigated. The microstructures of hot-bands were significantly refined by Nb addition, which led to the development of the cold-rolling texture in both the γ-fiber (<111>//ND-fiber) and the α-fiber (<110>//RD-fiber). Recrystallization was retarded by Nb addition, in particular, the growth of <110>//ND grains was retarded. However, the γ-fiber and {112}<110> grains developed during annealing even in the intercritical (α-γ) region. Consequently, the r-value increased as the content of Nb was increased due to the development of the intensity ratio of the <111>//ND texture to the <100>//ND texture, which is desirable for deep-drawable high strength steel sheets.

Abstract: The formation of ultrafine-grained structure in steels by various thermomechanical processings is reviewed from a metallurgical point of view. In the recent new type TMCP, ultrafine ferrite grains with a grain size of about 1μm are obtained when the austenite is heavily deformed at lower temperatures. In this case, dynamic phenomena such as dynamic recrystallization become prominent in the process. In the aging after heavy cold rolling of supersaturated matrix phase in two-phase alloys, the competition between the recovery or recrystallization of matrix phase and the precipitation of second phase occurs, resulting in various types of two-phase structures including microduplex structure. Microduplex structure is also obtained by annealing after heavy cold rolling of coarse two-phase structure in duplex stainless steel and high carbon steel. Recently, various severe plastic deformation processings, in which very large plastic strain over 4 is applied to the materials, have been developed to produce ultrafine grained materials with nanocrystalline and/or submicrocrystalline structures.

Abstract: The paper deals with the deformation and transformation behaviour of thermomechanically (TM) treated low alloyed Si-Mn TRIP steel. The aim of this work was to investigate the contribution of the factors governing the deformation and transformation process of conditioned austenite. Variation in strain and temperature parameters of TM treatment of TRIP steel samples resulted in formation of different complex microstructures. The deformation behaviour of TRIP specimens of different multiphase structures was tested in incremental neutron diffraction in situ tensile testing. It was proved that neutron diffraction technique is very convenient method for retained austenite (RA) transformation of the retained austenite with respect to monitoring of transformation quantification of retained austenite and rising internal stress in structural phases.

Abstract: The weathering of kimberlite ores leads to the weakening of the kimberlite structure, which allows the use of less intensive comminution procedures during the liberation of diamonds from the ore, resulting in a decrease in potential damage to the diamonds. The possibility to use iron- and sulphur-oxidising chemolithotrophic microorganisms as a way to accelerate the weathering of kimberlite ores was evaluated. It was demonstrated that the presence of sulphuric acid as well as ferric iron could result in changes in the clay component of these ores. The results provided promising evidence that weathering can be successfully accelerated, but showed that the degree of transformation varies between different kimberlite types with different mineralogical characteristics.

Abstract: Pure titanium foils were bent by laser forming and the effect of c-d transformation and history of heat treatment of specimen on bending angle was investigated. The thickness of specimens was changed from 40 to 100om, the length of them was 20mm and the width of them was 10mm. The specimens were annealed at 600-1100oC for 30 minutes in argon atmosphere. A 20W YVO4 laser was employed and laser power was changed from 2 to 16W. From the experimental results, when laser power was increased, bending angle also increased and it was dramatically changed at the laser powers occurring c-d transformation and melting. Bending angle increased as grain size increased and it jumped up when grain size exceeded the foil thickness and then became constant. Bending angle decreased by annealing after forming and degree of decrease was greater when the annealing temperature before forming was lower.

Abstract: Possible effects of stress on the movement of grain boundaries and phase boundaries are considered in terms of available driving forces and mechanisms. Examples of some of these effects are presented from the literature and new experimental results. Stress may influence the kinetics of transformation and also the microstructure and texture of the product material.

Abstract: The condition of a heat transfer surface with boiling is composed of three environmental components; high temperature, high pressure and water vapor. In general an autoclave or a high temperature loop system is used for maintaining the required condition. The thermodynamic relationship of chemical reactions states that the free energy difference (ΔG) is clearly dependent on the reaction temperature and independent of the reaction pressure if the reaction has no volume change: (∂ΔG/∂P)T = ΔV ~ 0. Based on the above relationship, the high-pressure term was removed from the environmental components of the heat transfer surface. A vacuum quartz capsule with a moisturized metal oxide powder was used to study the formation of nickel ferrite. The feasibility of this simplified method to simulate a heat transfer surface was confirmed by an analysis of the FT-IR spectra and XRD pattern during the transformation of a nickel and iron mixed oxide into nickel ferrite.