Materials Science Forum Vols. 638-642

Abstract: The mechanical properties of galvannealed dual phase steels bearing Si were investigated, and then the effect of Si on work hardening behavior during tensile deformation was discussed. Both tensile strength and yield strength were increased with increasing Si content, especially increasing ratio of tensile strength was larger than that of yield strength. On the other hand, the decrease of uniform elongation was very small with increasing Si content. Therefore, tensile strength and total elongation balance was improved by Si addition. High Si (1.2%Si) steel exhibited higher work hardening rate than low Si (0.01%Si) steel in all strain region. Dislocation cell structures were observed at 8% tensile strain in low Si steel, while dislocation cell structures were scarcely observed and instead, tangled dislocations were dominantly observed in high Si steel. Furthermore, high Si steel exhibited higher the increasing ratio of dislocation density in ferrite during tensile deformation than low Si steel. These results indicated that the formation of dislocation cell structure was retarded by Si addition, which led to the increase of work hardening rate and resulted in the improvement of tensile strength and total elongation balance.

Abstract: High deformability has been considered as a critical factor of ultra-high strength steel plates subjected to compressive, tensile, and bending deformation induced by large ground movements. In this paper, various dual phase microstructures consisting of soft ferrite and strong low-temperature transformation phases without deformation in the (austenite + ferrite) two-phase temperature region after controlled rolling were introduced and then the mechanical properties were discussed with emphasis on deformability such as yield ratio and uniform elongation. Ultra-high strength steel plates fabricated by a modified thermo-mechanical control process showed lower yield ratio of under 0.75 and higher uniform elongation of 5% as a minimum, as compared to commercial API X100 and X120 grade pipeline steels, without much sacrifice of Charpy impact properties because of an appropriate formation of soft ferrite and strong low-temperature transformation phases.

Abstract: The evolution of microstructure and texture in a Fe-22wt%Mn-0.6C% steel during plastic deformation was studied by interrupted tensile tests at plastic strain levels ranging from 5 to 40% at 10-3 s-1. The steel present in as-received condition a fully recrystallized austenitic microstructure and almost random texture. A microstructural analysis of deformed samples reveals the presence of thin bands. As no martensite was detected by X-ray diffraction, these striations must correspond to deformation twins. The amount of twins increases progressively with plastic strain. At lower deformation, these bands are parallel and start and terminate at the grain boundaries. As the strain level increases, the number of the twins band increases and they intersect each other. This evolution of the microstructure is accompanied by the appearance a fiber texture with the <111> and <100> directions parallel to the tensile direction

Abstract: The yield strength in austenitic stainless steels can be improved by cold rolling. Recently, it has been realized that a considerable further increase can be achieved through static strain ageing (SSA). The effect of SSA in four austenitic stainless steel grades was studied. The test materials were formerly cold rolled to three different reductions of 15%, 30% and 40%. Subsequently, the steels were aged at temperature range between 160 and 400 °C with ageing times from 15 to 15000 seconds. Owing to SSA, increments over 200 MPa in yield strength were observed, while elongation decreased only slightly or even improved by 1 to 2%-units. The influence of ´-martensite on the strength increase was apparent. The maximum strength increase with relatively small drop of elongation was achieved in the steels cold rolled to 30% reduction while approximately 50% of ´- martensite was formed. However, a small increase in the yield strength was detected even in steels cold rolled to 15% reduction and containing 0 to 2% of ´-martensite only. Therefore, SSA seems also to take place in the austenite phase. To clarify the reason for improvement of the ductility in the instance of strengthening, work hardening rates were determined and found to differ considerably between aged and non-aged structures. The activation energy of the SSA process determined was found to be almost equal to the activation energy of carbon and nitrogen diffusion in the austenite phase. A mechanism resembling the Suzuki effect was suggested as the main mechanism of the SSA process.

Abstract: The growth of columnar grains in fully recristallized IF steel during rapid heat cycles was examined experimentally. The heat cycles consisted of continuous heating with a rate up to 1500°C/s followed by water or air cooling. The employed heating method, as well as the geometrical form of the samples, enabled to obtain the temperature gradients up to 2000°C cm-1. Moreover, temperature measurements and recordings with the aid of ultra-rapid infrared pyrometry made it possible to determine the characteristic temperatures of phase transformations taking place during heating and cooling periods. The main key parameters of the columnar growth, including temperature gradient and the displacement rate of isotherms corresponding to ferrite-austenite and austenite-ferrite phase transformations could also be examined. The results show that the growth of columnar grains already starts at the heating stage at the ferrite/austenite interface moving against the temperature gradient. During the air cooling period, the growth is taking place according to the temperature gradient, together with the austenite/ferrite interface displacement. It was suggested that columnar-like morphology development occurs according to a selective growth mechanism.

Abstract: Blast furnace work involves the flow of enormous volumes of raw materials. Modifications of the blast furmace operation parameters can bring about savings connected with materials consumption and also a reduction of production costs. The continuous technical-economic analysis of this process enables changes in the process to be observed by means of simple indexes. In this article, a technical-economic analysis of the blast furnace process is presented. It is based on the results of a Polish blast furnace with an overall capacity of 3200m3.

Abstract: Quickness of fracture of Al oxide film is important especially for vacuum free Al/Cu bonding. Displacement amount and rate relate closely with fracture of oxide film and create clean surface. Displacement history during bonding process under static bonding load was measured for displacement control systematically. Displacement history under static load consists of three stages with creep and liquefaction. Higher bonding load increased displacement rate of each stage and decreased the thickness of Al oxide film grown by air heating. Suitable hearing time for bond also was shortened by increment of bonding load. Two types of displacement control were designed from these experimental results. One was the accelerated displacement rate control, another was the cyclic displacement control. Fracture of Al oxide film and liquefaction of bonding surface occurred at lower displacement than static load by each displacement control.

Abstract: Effect of liner laser irradiation on bending properties of thin high carbon steel plate was investigated in this study. Laser-transformed zone included solidification structure and quenched zone by laser irradiation was observed in cross section area of specimen by double-etching. Shape of laser-transformed zone changed significantly with changing of heat input. Deformation after laser irradiation occurred by the excess of heat input. Average area ratio of laser-transformed zone for normal bending specimen without deformation was cleared at each thickness. Sensitivity for laser deformation increased with decreasing of thickness. Three points bending test was executed to obtain Flexural load-deflection curve. Bending property of thin steel plate improved by laser liner irradiation and flexural load per unit deflection increased by suitable laser irradiation condition. Absorb energy of bending test also increased by existence of laser-transformed zone.

Abstract: Lath-shaped upper bainite structures play a very important role in many high-strength steels (HSSs) and ultra high-strength steels (UHSSs). Although bainite transformation is strongly affected by the initial structure, the effect of the second phase in a multi-phase structure is yet to be clearly understood. It is significant for the advancement of UHSS to study this effect. The aim of this study is to clarify the effect of martensite, which forms before bainite, in Fe-0.2C-8Ni alloy. The bainite transformation from an austenite and martensite dual-phase structure is faster than that from single-phase austenite and the nucleation of bainitic ferrite laths are accelerated around martensite. This effect of martensite on bainite kinetics is equivalent to that of polygonal ferrite when their volume fractions are almost the same. This suggests that the boundary between martensite and austenite is a prior nucleation site of bainitic ferrite. Martensite also affects the crystallographic features of bainite. The orientations of bainitic ferrite laths tend to belong to the same block with martensite adjacent. This tendency intensifies with an increase of the transformation temperature of bainite, resulting in the formation of huge blocks consisting of bainitic ferrite and martensite laths at high temperatures (693K and 723K). In contrast, at a low temperature (643K), bainitic ferrite laths belong to same packet as martensite and have several orientations. This change of crystallographic features with transformation temperature can explain with the driving force of the nucleation of bainitic ferrite.

Abstract: The mechanical properties and microstructures of alternative low carbon TRIP-aided steels in which manganese contents mediate between conventional low-alloyed TRIP-aided steels and TWIP steel have been investigated. A variety of microstructures, from a single austenite phase to multiple phase mixtures, was attained according to chemical compositions as well as heat treatment schedule. By means of reverse transformation of martensite combined with controlled annealing, a remarkable grain refinement being responsible for stabilization of austenite could be achieved. In case of the duplex (+ ) microstructures in 6Mn and 7Mn alloys, large amount of retained austenite more than 30 % contributed to substantial improvement of ductility compared to the conventional TRIP-aided steels having similar tensile strength level. In nearly single austenitic 13Mn alloy, the annealed sheet steel exhibited high tensile strength of 1.3 GPa with sufficient ductility due to the stain induced martensite transformation of fine grained austenite.