Papers by Author: Hao Yu

Abstract: Two X70 pipeline steel plates with different finishing cooling temperature showed different DWTT indexes. In order to investigate the influences of microstructure and texture on toughness EBSD and X-ray were employed. Plates with lower toughness had higher low angle boundary proportion than the other, which is different from the previous research. Then the macro textures of the two plates were obtained by the X-ray diffractometer. It was shown that the ‹110›//ND were dominant in both of the two plates. The effects of several components on the toughness were analyzed. This work shows that the {112}‹110› component is the favorable texture benefiting the toughness. ‹111›//ND has direct influence on the yield stress and toughness.

Abstract: The continuous cooling transformation (CCT) behaviors of two vanadium (V) micro-alloyed steels with different Nitrogen (N) levels were investigated under the undeformed and deformed conditions, respectively. It is found that the austenite-bainite transformation is suppressed during dynamic CCT process due to hot deformation. For V-N steel, the transformation of austenite-proeutectoid ferrite and austenite-acicular ferrite (AF) occur under a wider range of cooling rates.

Abstract: Orientations distribution between grains of two high grade pipeline steels were investigated by electron back-scattered diffraction (EBSD). Then the percentage of low-angle grain boundaries was studied qualitatively to analyze the effect of low-angle grain boundaries on the yield-strength ratio of high grade pipeline steels. From the mode of coordinate deformation and the ability to resist deformation by the grain boundaries, the results show that when the effective grain size are almost the same, the pipeline steel which has the smaller percentage of low-angle grain boundaries, the larger difference between the yield strength and tensile strength, which makes the yield-strength ratio of pipeline steel lower.

Abstract: The evolution of microstructure and texture due to recrystallization was investigated in cold rolled interstitial free (IF) steel. Samples taken from the cold band of a 0.07%Ti bearing IF steel with total cold rolling reduction of 75% were investigated by using optical microscopy, X-ray diffraction and electron back-scattered diffraction etc. The aim of this study was to obtain a real picture of the formation of the recrystallization texture of IF steel, which would contribute to proper texture control for improving deep drawability. The mechanism responsible for the evolution of texture is discussed experimentally from four aspects. First of all, the microstructure of partially annealed cold rolled specimen is obtained. The recrystallized grain form earlier in more darkly etched regions from the micrographs, which belong to the ND//<111> fiber components. In addition, the fractions of the {111}<110> and {111}<112> components increase slowly during the early stage of recrystallization, because the γ-fiber recrystallized texture is growing, while at the same time the γ-fiber deformed texture is being consumed. Moreover, The texture formation has been discussed taking into consideration of the stored energy and the misorientation between the orientation of the recrystallized grain and that of the surrounding deformed matrix. The work indicates that the recrystallized grains, which migrate into the deformed grains, are mainly with the high misorientation angles. The large percentage of the recrystallized grains, whose misorientation angles with deformed grains exceed 15°, are corresponding to the {111} transformation texture. Nucleation first starts at colonies that have the highest stored energy of deformation, which has the same orientation as the deformed grains. By analysis, the formation of recrystallization texture was well explained by oriented nucleation mechanism rather than by selective growth mechanism.

Abstract: The microstructure and properties of aging at room temperature of dual phase steel produced by continuous annealing were investigated by using mechanical property test, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). In order to know the mechanisms of aging of dual phase steel produced by this process, the relative theories of dissolution and diffusion of interstitials, dislocation and precipitation are proposed and discussed. From analysis, the results showed that dual phase steel produced by continuous annealing had high strain aging resistance after overaging.

Abstract: Formation of austenite strongly influences the microstructures and mechanical properties of dual phase steels. In present work, austenization process during intercritical annealing was studied in a Fe-C-Mn steel using Gleeble-1500 thermal simulator and quantitative microscopy. The experimental results show that austenite formation is separated into three different stages: (i) growth of high carbon austenite into pearlite rapidly until pearlite dissolution is completed; (ii) slower growth of austenite into ferrite; (iii) very slow equilibration between ferrite and austenite. The thermodynamic and kinetic analyses show that growth of austenite into ferrite is controlled by carbon diffusion in austenite in the primary stage and manganese diffusion in ferrite in the subsequent stage because diffusion coefficient of Mn in ferrite is several orders of magnitude smaller than that of C in austenite. The slow final equilibration between ferrite and austenite is obtained by manganese diffusion through the austenite. Based on the analysis, one dimensional diffusion model of intercritical austenization was developed and solved using finite volume method on the assumption that solute flux was local balance at interface, and the kinetics calculated was compared with experimental results. Simulated results indicate that growth of austenite reaches paraequilibrium in about one second, but remains thousands of seconds to reach final equilibrium. Simulated concentration profiles show that manganese atoms transferred from ferrite congregate in austenite near phase interface, which is consistent with the experimental phenomenon.

Abstract: Different cooling parameters, including the initial cooling temperature, finishing cooling temperature and cooling rate, have a significant effect on the final microstructures and properties for pipeline steels. In present work, Gleeble-1500 thermal-mechanical simulator was used to investigate the microstructural evolution of X70 pipeline steels under different cooling conditions, and the microstructures obtained were analyzed using optical microscope and transmission electron microscope. The experimental results showed that when the initial and finishing cooling temperatures are controlled in the range of 740~760°C and 500~520°C respectively, the microstructure of X70 pipeline steels reveals a proper content (about 12%) of pre-eutectoid ferrite besides acicular ferrite and M/A (Martensite/Austenite) island, which can guarantee an optimum combination of strength and ductility. In contrast to the conventional way using CCT curve, this methodology shows a higher accuracy and operability, and used in industrial production to achieve good effect.