Materials Science Forum Vols. 561-565

Abstract: Continuous casting is the essential process converting liquid steel to solid in the form of slabs or billets/blooms in the steel plant. The economy and quality of the steel products are greatly dependent on how successfully the continuous casting is performed. New technologies have been actively developed in the process during the last decades in order to increase the productivity and, therefore, to decrease the operational cost. Since its first commissioning of a slab caster in 1976, POSCO has constructed a number of continuous slab, bloom and billet casters including a thin slab caster not only for plain carbon steels but for stainless steels. Through the operation of various types of continuous casters for more than 30 years so far, POSCO has steadily developed fundamental technologies and operational know-how and achieved the equipment innovations to improve the surface and internal qualities of cast products as well as to extend the productivity of continuous casters. Furthermore, POSCO has deepened the basic understanding on the solidification phenomena of liquid steel and also accumulated the engineering backgrounds to design the most optimal continuous casters. It has also devised the indispensable and auxiliary equipments and the key technologies to control the process precisely and efficiently in order to guarantee the quality and productivity. An innovative technology under development is the POCAST process, where controlled amount of the pre-molten mold flux instead of conventional powder mold flux is continuously fed into free surface of molten steel through the plunger-type feeding system from the flux melting furnace. In order to prevent the molten flux from freezing at the meniscus, a reflective insulation cover is installed, leading to the suppression of thermal radiation from the molten steel and flux. It is generally understood that, as casting speed increases, the occurrence of breakout increases since mold lubrication becomes insufficient due to the lack of mold flux flow from the meniscus into the solid shell/mold boundary. However, by utilizing the especially composition controlled pre-molten flux, it becomes possible to eliminate the formation of slag bear in the mold. Therefore, the mold flux consumption rate is increased even at the reduced oscillation rate & stroke and more importantly, the mold flux infiltration becomes more uniform throughout the boundary between the mold and the solidified shell. This consequently results in drastic reduction of the formation and depth of the oscillation mark and the occurrence of surface hooks without increasing the possibility of breakout, as has been proved in the casting trials carried out with the 10 ton pilot slab caster in Pohang. A key trend in the development of the continuous casting process is to reduce the thickness of cast products. Examples include thin slab casting and strip casting. In the thin slab casting process, a major drawback is the relatively low casting speed and, as a result, the inefficient equipment layout in the plant where two casters are connected to a hot rolling unit. The drawback could be resolved if the casting speed exceeds a certain limit. At the high casting speed, the productivity of casting becomes equivalent to that of hot rolling, and the thin slab casting plant is to be designed so that one strand

Abstract: The basic characteristics of TiNi-based and Ni-free Ti-based shape memory alloys are reviewed. They include the crystal structures of the parent and martensite phases in both the alloys, the recoverable strain associated with the martensitic transformation, the transformation temperatures, the temperature and orientation dependence of deformation behavior, etc. The sputter-deposited Ti-Ni thin films are also reviewed briefly because of their possibility of expanding into micromechanical system applications as the most powerful microactuator.

Abstract: The deformation and recrystallization behaviour of a range of Nb microalloyed steels has been studied using hot torsion. This work focuses on the change from strain dependent to strain independent recrystallization behaviour as a function of the alloy content, initial microstructure and deformation conditions. It is found that there is a complex interaction between deformation, recrystallization and strain induced precipitation, which has significant implications for controlled rolling in hot strip and plate mills. The data also revealed that the pre-existing precipitates did not influence the behaviour of post deformation softening.

Abstract: The phase transformation rule, microstructures and properties of JG590 high strength steel produced in Jinan Iron and Steel Co. ltd. have been investigated in this paper. When the chemical composition of steel are given, the cooling rates after finished rolling affect on the properties of steel greatly. The yield strength and tensile strength increasing, the elongation and reduction of area decreasing as increasing of cooling rates after rolling. The main cause is due to appearance and increasing of Bainite and Martensite other than Ferrite and Pearlite in room temperature. The finished rolling temperature have distinct effects upon the mechanical properties of steel plates. Finished rolling at different temperature with the 0.5°C/s cooling rate, the tensile strength vary in 599-698MPa, the yield strength changed from 412 MPa to 536MPa. The elongation is between 30.4-40.5%. But when finished rolling at different temperature with the 2.0°C/s cooling rate, the tensile strength vary in 747-784MPa, the yield strength changed from 441 MPa to 601MPa. The strength index can both meet the requirements of employ. But the elongation is only 18.7-24.5%. This is related with production of lots of Bainite microstructure more than 2°C/s cooling rate. In the procedure of manufacture of JG590 high steel, the quickly cooling rate should be avoided to keep suitable microstructure and good elongation and toughness.

Abstract: The effect of initial grain size on the recrystallization behaviour of a type 304 austenitic stainless steel was investigated using hot torsion. Refinement of the initial grain size to 8 μm had a considerable effect on the flow curve shapes, peak and critical strains and also on DRX kinetics, compared with an initial grain size of 35 μm. Microstructural investigations using EBSD suggest a transition from conventional (discontinuous) to continuous dynamic recrystallization with decreasing initial grain size. Also, there was a moderate effect of initial grain size on the DRX grain size.

Abstract: Effects of tempering temperature and shot peening on fatigue behavior of SAE 1045 steel are reported. Tensile and fatigue property measurements are reported for samples tempered and shot peened at 300 and 550°C. Microstructural studies and fracture surface observations were carried out by optical and scanning electron microscopes. The experimental results showed that, fatigue strength increases with decrease of tempering temperature and shot peening treatment. SEM analyses indicated that all three stages of fatigue fracture affected by tempering treatment whilst shot peening treatment only have an effect on the crack initiation stage.

Abstract: The samples of V-N microalloyed mild steel were obtained in duo mill with the normal deformation rate and the normal rolling temperature except final rolling temperature, which is at 780°C, 730°C and 680°C respectively. The tensile test was carried out and the microstructure was observed with optical microscope. It was found that the mechanical properties improved dramatically compared with normal process, the final rolling temperature is more than 900°C. The main reason is the fine grain size and second phase particles. With the final rolling temperature decreasing, the mechanical property further improves until the final rolling temperature of 700°C.

Abstract: In this study, a TMCP was attempted to obtain a low carbon high strength Nb-Ti microalloyed dual phase with low yield ratio at Baosteel. Optical and scanning electron microscopy was utilized to analyze microstructures. A mixed fine microstructures consisting of acicular ferrite and martensite/austenite was observed. The austenite maintained a small grain size during reheating in the temperatures ranging from 1100°C to 1250°C. The growth of austenite was strongly retarded by Nb and Ti nitride and carbide. The higher end cooling temperature (650°C) had much influence on the refinement of microstructures, especially the length of ferrite packets, whereas the lower temperatures (600-500°C) had little effect on the microstructure refinement.

Abstract: The comparative effectiveness of solute Nb and NbC particles at impeding grain boundary motion is treated theoretically. It is shown that, for a steel containing ~0.05 at% Nb (~0.1 wt%Nb), under typical recrystallization conditions, solute Nb is more effective in the ferrite, whereas in the austenite, depending on the exact recrystallization temperature, either solute Nb or NbC precipitates may be more effective.