Advanced Materials Research Vols. 690-693

Abstract: The effect of RE on modifying inclusions of H13 die steel was studied by metallographic examination, SEM and electron spectroscopy. Thermodynamic calculation was used to analyze the formation of RE inclusions in H13 die steel. The result shows that sulfide inclusions are modified to round RE complex inclusions after adding RE to H13 die steel.

Abstract: By using the Gleeble - 1500 hot simulation test machine we studied 20#, 60Si2Mn, 9Cr18MoV the three kinds of low, medium and high carbon steels to observe the dynamic recovery and recrystallization of ferrite while in the process of thermoplastic deformation. We calculated the hot deformation activation energy of each kind of steel by combining the stress-strain curve we got in the experiment and the theoretical model of Z parameter. It turns out Q_9Cr18MoV<Q_20#<Q_60Si2Mn. In the meanwhile, microstructure observation through transmission electron microscope shows that the dynamic recrystallization of ferrite is more likely to happen in 9Cr18MoV steel than in 20# steel and in 60Si2Mn steel. These results indicate that the dynamic recrystallization of ferrite is not only determined by stacking fault energy but also closely related with the strain storage energy release degree.

Abstract: The oolitic hematite with a lower iron content and a higher phosphorus content will be a potential source of raw iron ore for the Chinese iron and steel industry. The magnetization reducing roasted and magnetic separation is a practical beneficiation process. The results show that the iron content in the concentrate can be increased to 57.63% from 45.95%, the iron recovery up to 73.12%, when the parameters are as follows, the roasting temperature is 900°C, the residence time 70 min, the ratio of reducing agent 5%, the grinding time 7 min, and the magnetic field strength 0.15T. The phosphorus content in the concentrate can be reduced to 0.23% from 0.39% by means of acid leaching.

Abstract: In this work, a new steel with excellent corrosion resistance was developed for cargo oil tanks of crude oil tankers to substitute protective coatings. Its corrosion properties were investigated by immersion test, SEM and potentiodynamic polarization techniques. The results indicated average corrosion rate of the steel was 0.22 mm/year, which countered for ca. 1/5 of accepted criterion (C.R. 1 mm/year) for corrosion resistant steels. This was due to the formation of a thin (10-15 μm), homogeneous and compact corrosion product with high polarization resistance on the surface of the sample.

Abstract: Results of uni-axial tensile loading of three automotive steels at different strain rates (0.0011–3200s-1) are reported here. Quasi-static tensile tests were performed under the strain rate of 1.1×10-3 s-1 using an electromechanical universal testing machine, whereas dynamic tests were carried out under the strain rate in the range of 1100 to 3200 s-1 using a Split Hopkinson Tensile Bar apparatus. Based on the experimental results, the material parameters of widely used Johnson–Cook model which described the strain rate and temperature-dependent of mechanical behaviour were determined. The experiments show that strain-rate hardening is superior to thermal softening: yield stresses, tensile strength, deformation, and energy dissipation increase with the strain rate from quasi-static tests to dynamic tests. The Johnson–Cook model can describe the behaviour of these steels and provides the opportunity to study the material and structural response.

Abstract: Effect of secondary cooling on non-oriented electrical steel strips was investigated. The 2.0 mm thick cast strips contain two compositions were produced by twin-roll casting process, cooled in the air or cooled by spraying water. The microstructure was observed by optical microscopy, and EBSD was used to characterize the texture of the cast strips. The results showed that air-cooling cast strips have uniform and equiaxed grains with average size of 250 μm. The microstructure of the water-spraying cast strips compose of most equiaxed grains and a small number of abnormal big grains. At the same time, the secondary cooling rate mildly affects the cast texture strength but has no influence on the texture type.

Abstract: Distribution of residual stress in hot rolled low carbon martensite high-strength steel strip was measured by means of blind-hole method in the steel before and after tempering. The hot rolled low carbon martensite high-strength steel strip was tempered at 450°C, 500°C, 550°C or 600°C. Before tempering, the value of the residual stress along the width direction is maximum at the edge, intermediate at the center, minimum at the 1/4 of the strip. The figure of the residual stress distribution along the width direction is like the shape of the letter M. Residual stress of the strip is reduced after tempering. When tempering at 450°C or 500°C, evolution of residual stress is caused by changes of thermal stress. Distribution of residual stress becomes gentle. With tempering temperature increasing, distribution of residual stress is reversed, because evolution of thermal stress and phase transition stress changes in different time.

Abstract: The hot ductility behavior of 800MPa ultra high strength niobium containing steels has been investigated with Gleeble3500 hot simulator. Tensile test was carried out at 650°C1300°C at a constant true strain rate of 0.001 s¹. Experimental results showed that steel A has quite well ductility, and the brittle zone is narrow. Due to pro-eutectoid ferrite film formation along the prior austenite grain boundary at 650°C-750°C, samples showed a loss of hot ductility, and the fracture morphology of the specimens was brittle intercrystalline fracture, indicating the third brittle zone of the steel. However, the third brittle zone can be avoided during continuous casting, if the temperature of straightening could be kept over 800°C.

Abstract: In this paper, firstly, the morphology characteristics of aggregated Al₂O₃ inclusion were quantitatively described by fractal dimension and hydrodynamic radius. Then, a general collision-agglomeration model was established to describe the collision and agglomeration process of Al₂O₃ inclusions with different morphology characteristics. The results showed that: in the collision and agglomeration process of Al₂O₃ inclusions with different morphology characteristics, the number density of monomer inclusion decreases with time, but that of new aggregated inclusions, firstly increases to the maximum value, then decreases sharply; the smaller the fractal dimension of new aggregated inclusion, the more rapid decrease of number density of monomer inclusion and the larger agglomeration rate of new aggregated inclusions. so in order to accelerate the removal rate of micro inclusion particles, it is a viable way that keeping the fractal dimension of new aggregated inclusions at an appropriate value, not too small.

Abstract: This paper introduced the primary research on the method of using limestone instead of lime for slagging during BOF steelmaking. The study showed that the heat is sufficient for limestone to completely replace lime on the condition that the ratio of hot metal to ferrous scrap is adjusted properly. According to thermodynamic calculations, the partial pressure of CO₂ at the hot metal surface at the early stage of converter steelmaking is close to 0 and nearly all the CO₂ can react with elements in hot metal, producing CO and reducing O₂ consumption. Slag with high oxidizability, high basicity and relatively low temperature can be formed quickly by using limestone, which is advantageous for dephosphorization. Industrial tests showed the endpoint Lp rises with the proportion of lime replaced by limestone increasing. CO₂ volume expands by 4000~30000 times when limestone is heated at extremely high speed in converter, resulting in the fragmentation of some parts of limestone and the increase of the contacting area of limestone and slag. The unreacted core model with surface exfoliated was introduced.