Advanced Materials Research Vols. 284-286

Abstract: The effect of 0.06%Nb on the microstructure and mechanical properties of grain boundary allotriomorphic ferrite (F_GBA) / granular bainite (B_g) air-cooling bainitic steels has been investigated in this paper. The results indicate that the steel acquires superior mechanical properties by adding 0.06%Nb. Compared with Non-Nb steel, the addition of 0.06%Nb increases the tensile strength and yield strength about 37.1% (From 780MPa to 1070MPa)and 26.6%(From 557MPa to 705MPa) respectively, remaining 18.3% elongation and 97J toughness. The addition of 0.06%Nb not only promotes the nucleation of intragranular ferrite but also refines the allotriomorphic ferrite grain , both of which in turn contribute to the refinement of granular bainite cluster including its ferrite platelets and M-A islands. Under the synthetic roles of the microstructure refinement and precipitation strengthening, 148MPa yield strength improvement has been acquired in the low carbon air-cooling bainitic steel by the adding of 0.06%Nb.

Abstract: Ovality defect of round bloom castings is the haunting defect of demanding grades steel, which can be attributed to excessive mechanical stress imposed by the roll containment especially at the unbending region. A 2-D elastic-plastic FEM model has been developed for the quantitative analysis of the effects of hot strand pressures on the ovality deformation and the contact normal force distribution. It is shown that higher friction force and smaller reduction deformation can be expected through the adoption of grooved rolls for given hot strand pressures as compared with plain rolls. For the determination of whether the given hot strand pressures can meet the requirement of strand downslide control in the caster, the resistances generated in the mold, secondary cooling zone and unbending zone are analyzed. Accordingly, an iterative algorithm has been presented to modify the hot strand pressures for given caster. A set of modified hot strand pressures has been computationally determined, which has been proved to be safe in production for casting round bloom with diameter up to Φ400mm with better roundness and less roller mark.

Abstract: There are many serious problems are directly associated with the flow pattern in the mold in the continuous casting of steel process. The nozzle flow plays an important role in determining the mold flow pattern and to minimize casting defects. In this work a new type nozzle designed for a slab casting mold is developed. The 3-D steady k-ε finite-volume model has been applied to study the turbulent flow in nozzle and mold, which predict velocities agree with the impeller measurements in the water model well. The results show that nozzles with a mountain bottom are more impressionable to problems from asymmetric flow and large fluctuation on the mold surface. The impact of liquid at wave bottom nozzle can suppress the jet flow effectively and reduce the velocity variation.

Abstract: Flow control devices (weir and dam) in a continuous casting tundish are very important to the flow field, which influences the temperature uniform and the inclusion floating. In this work, the weir and dam were firstly optimized through numerical simulation and water simulation synthetically by orthogonal optimization tests. And the optimal parameters showed that the distance from upper weir to inlet was 1000 mm, the distance of upper weir to tundish bottom was 150 mm, the distance from upper weir to dam was 600 mm, and the height of the dam was 320 mm. Then the effect of different arrangement holes on the dam was discussed through RTD curve and velocity field under the optimum flow control device. And it revealed that the hole influenced the flow pattern in that area obviously, a dam with two holes could get a better flow field.

Abstract: Based on difference of Mn，Fe and Mo vapor pressure，Fe-Mn，Fe-Mo alloys were added into induction furnace and steelmaking dust was collected during smelting process，whose granularity was analyzed by granulometer，microscopic structure and energy spectrum were analyzed by SEM to study characteristics of dust and ulteriorly to explore mechanism of dust generation. It was showed that the mechanism of dust generation was “evaporation theory” and “bubble theory”，dust lied in evaporation had uniform distribution，granularity and element distribution，Mn/Fe ratio of dust was larger than that in molten pool. However, dust caused by spitting or bubbles from molten metal surface had remarkable aggregation，nonuniform granularity and element segregation，Mn/Fe ratio was equal to that in molten pool.

Abstract: The effects and mechanism of cerium, lanthanum and cerium-lanthanum alloys on microstructure and the impact toughness of high-carbon steel were studied in the present work. For high-carbon steel, the state and the content of RE were measured, and the effects and the mechanism of RE on sulfide inclusions, microstructure and the impact toughness of steel were determined. With increasing the RE addition, the sulfide inclusion can be changed from strip-like, spindle, ellipsoidal and spheric in shape. Strip-like sulfide disappears in high-carbon steel with the RE addition being 0.05%. A small quantity of RE can fine the austenitic grain, decrease the height of the SKK peak of the internal friction curve, and improve the impact toughness of high-carbon steel evidently. And the effects of lanthanum on fining the austenitic grain and improving the impact toughness is the largest, next to that of cerium-lanthanum alloys, and that of cerium is the least, which can be verified by the internal friction experiments.

Abstract: Under different deformation temperatures and strain rates, the thermal deformation process of Cu-P weathering steel was studied using Gleeble1500D-type thermal simulator. After the high temperature rheopectic curves at different conditions and thermal deformation equation was got, the dynamic recrystallization diagram of the steel was plotted. The results show that the thermal deformation activation energy of the steel is 345kJ/mol, and the dynamic recrystallization diagram of the steel consisits of three parts as completely dynamic recrystallization zone, partially dynamic recrystallization zone and non-dynamic recrystallization zone.

Abstract: The metallurgical properties of oxidized pellets are of great importance to achieve high efficiency and smooth running of blast furnace and gas-based direct reduction shaft furnace. In this study, the new method of adding B-Mg compound additive has been put forth to improve metallurgical properties of pellets. The effects of adding B-Mg additive on the strength of green balls, cold compressive strength, reduction swelling, and the strength after reduction and cooling of oxidized pellets are investigated through the experiments and microstructure analysis. The results revealed that, the B-Mg additive has not-so-remarkable effects on the properties of green balls. The rational addition amount of B-Mg additive is 0.6 %, and B-Mg additive make it feasible to reduce roasting temperature of the pellets. High temperature properties show remarkable improvement with the increase of B-Mg additive amount, the RSI decrease from 14.7% to 7.17%, and the strength of pellets after reduction and cooling increase from 162.5 N to 650.8 N when the addition amount increases from 0 to 0.6%.

Abstract: In this article, the microwave magnetizing roast process of three kinds of Bayan Obo ore of different Fe-grades were studied experimentally, where microwave as the heat source and the activated charcoal as reductant. The reduction characteristics of hematite changed into magnetite by microwave magnetizing roast were studied analytically under different experimental conditions, where magnetisability and percent reduction of roasting ores as measure.It showed that under the optimum conditions of microwave roasting of hematite (the theoretical carbon content, the microwave voltage of 220V, the temperature of 650 °C and the heat preservation time of 10min), magnetisability and percent reduction of hematite were best , close to the theoretical value of 2.33 and 11.11% respectivly. And after the first weak magnetic separation of microwave roasting ores , the recovery of iron were all above 90%, which indirectly showed that the microwave reducing roasting magnetization effect were very good under the conditions. The Fe-grades of iron concentrate may achieve above 60% after the second magnetic separation ,which met the requirement of iron-making.

Abstract: The effects of process parameters on microstructural evolution and grain refinement are determined in a Nb-Ti microalloyed high strength dual phase steel. With the increase of cooling rate, final microstructures change from a mixture of acicular ferrite (AF)+martensite/retained austenite (M/A) to conventional bainite (CB)+M/A. Accordingly, the morphology of M/A constituent changes from an equiaxed island in AF to an elongated interlath in CB. The length and width of bainite packets become smaller with the increase of cooling rate and the decrease of deformation temperature. The length of individual bainitic ferrite plates within the packets become smaller with the increase of cooling rate and the decrease of deformation temperature, whereas the thickness of them varies slightly with them. The optimized relaxing time on grain refinement is 60 s. The reheating temperature, reduction ratio and interrupt temperature has no obvious effect on the formation of dual phase of acicular ferrite or bainite and M/A.