Papers by Author: Di Tang

Abstract: In order to control the ratio of nano/ultrafine structure grains of warm/cold rolled 316L stainless steel after annealing, the influence of deformation amount and temperature on martensite content and microstructure was investigated, and a model of the content of stain-induced martensite and deformation amount and temperature was established. Results showed that the content of stain-induced martensite was nonlinear with deformation amount, but with an incubation period. And it’s generally exponential. Martensitic transition occurred in large deformation stage. 58.23% strain-induced martensite was formed when deformation amount was 80% at 200°C. The content of martensite is the most significant factor that affects austenite grain size after annealing. With more strain-induced martensite, the average austenite grain size decreased, and the uniformity of grain size was improved, which was generally monotonous.

Abstract: The surface of hot-dip galvanized high strength low alloy (HSLA) steel easily occurs orange peel in the deformation process. On the other hand, the defects possess a specific directivity and sits at approximately a 45-degree angle to the sheet steel rolling direction. The microstructures and properties of steel specimens with the orange peel defects and the normal were analyzed, which results showed that their microstructures consist of ferrite and few granular pearlite. The yield point elongation of the HSLA steel resulted in the orange peel defects on the surface of sheet stamping and it is associated with skin rolling and stretch rolling process. Further studied on the fine microstructures by means of SEM and electron back scatter diffraction (EBSD) techniques, which was apparent for the defect steel that the orange peel defects were resulted from weak favorable {111} texture might be the key factors aggravating the formation of orange peel defects. It can be concluded that the formation of Cottrell atmospheres caused the yield point elongation by the interaction between dislocation and diffusive solute atoms as basic reason and the directivity of the orange peel defects was related with the LUDERS slip forming. The yield point elongation can be eliminated to avoid the orange peel defects beyond to 1.8% skin-rolling and stretch rolling method with an appropriate annealing technology.

Abstract: The microstructures of CR600TR steel were characterized using optical metallography, SEM, TEM and EBSD techniques, especially the transformation behavior of retained austenite and fracture mechanism of TRIP steel under stress-strain were studied in detail. It is discovered that there is marked selectivity for those retained austenite grains in their transition sequences: they are even dependent on the extension paths of stress-strain in steel and that curvatures of grain-boundaries which the retained austenite grains lie in.

Abstract: The isothermal transformation of wustite (FeO) in the range of 650~350°C was studied using Gleeble 3500 thermal simulated test machine, electron back-scatter diffraction (EBSD) and scanning electron microscopy (SEM). A wustite layer was initially formed by oxidizing low carbon micro-alloyed steel in air at 880°C for 20 seconds. The layer was then isothermally annealed at 650-350°C in argon for up to 10000 seconds. The transformation of FeO was comprised by a proeutectoid reaction and a eutectoid reaction. FeO initially transformed to Fe3O4 by proeutecioid reaction and then the retained FeO transformed to lamellar mixture Fe+Fe3O4 by eutectoid reaction. The proeutectoid and eutectoid reactions both followed a C-curve trend and the nose temperatures were 475°C and 425°C, respectively. An isothermal transformation (IT) diagram was summarized based on our results.

Abstract: Four kinds of X100 grade pipeline steels containing different chromium content were designed and corrosion behavior were studied in order to confirm the influence of Cr on the properties of high grade pipeline steel. The corrosion experiments demonstrated that Corrosion rates of Four steels in the experiment decrease with the increasing of Cr content, and the scales on the four steels have a two-layer structure under the corrosion of CO2 and H2S. The outer layer is mainly composed of FeS or FeS1-x and the inner layer consisted of FeCO3. Cr enriches in the inner layer and Cr content of the inner layer increases with the Cr content in matrix. The Cr enrichment enhanced the compactness of the scales further hindered the diffusion of ions from liquid to the surface of steel. The corrosion scale with Cr(OH)3 is anion-selective and reduce the amount of anion reaches metal surface, thus reducing corrosion rate.

Abstract: The fatigue limit and S-N curve of Q500q bridge steel were obtained by high-cycle fatigue test. The experimental results show that the fatigue limit of the experimental steel is 552.5MPa at room temperature with stress ratio R=0.1.There are some differences from the traditional fatigue fracture of high-cycle fatigue, the experimental steel’s rapid propagation area shows the morphology feature of both plastic fracture and brittle fracture. From analyzing the fatigue facture, the morphology feature of plastic fracture is mainly caused by the high fatigue limit. The little size (2~4μm) of inclusions in the experimental steel and the acicular ferrite’s microstructure feature of sub-lath structure with high-density dislocation reduce the crack initiation. And that’s the main reason why the steel has such high fatigue limit.

Abstract: The main purpose of this study is to develop a numerical model to describe the static recrystallization kinetics in X70 pipeline steel. In order to investigate the static softening behavior, double-hit compression tests were performed on a Gleeble-3500 thermomechanical simulator over a temperature range of 950-1150°C. The interpass time varies from 1-500s. Different values were applied to estimate effects of strain rate and deformation on static recrystallization. The results show that factors such as high temperature, long holding time and lower strain rate will lead to the inclination of softening behavior. The static softening activation energy was obtained. The good agreement between the predicted values and experimental results indicated the validation of the developed model of this study.

Abstract: Effects of continuous annealing process on microstructure and properties of Si based cold-rolled TRIP Steel were studied. The results show that the TRIP effect is more obvious on the condition of 800°C with 120s annealing process and 400°C with 520s aging treatment than others progress. The yield strength of the tested steel is 405MPa, tensile strength is 670MPa, elongation is 32% and strain hardening exponent is 0.230. The microstructures are ferrite, bainite and retained austenite and the volume fraction of retained austenite for the tested steel is 9.20%.

Abstract: The high-temperature deformation resistance of X120 pipeline steel was studied under different deformation temperature and different deformation rate through the hot compression test on the Gleeble-3500 thermal/mechanics simulation test machine. The influence of deformation degree, deformation temperature and deformation rate on deformation resistance was thoroughly investigated. The deformation resistance of X120 pipeline steel increased slowly when deformation degree was higher than 0.2. With the increase of deformation temperature, the work-hardening effect was weakened, so the deformation resistance decreased. And the deformation rate had dual influences on the deformation resistance, including the effect of temperature and time. Based on the experiment data, the parameters in the mathematical model were regressed by using SPSS (Statistic Package for Social Science), and the mathematical model of the deformation resistance of X120 pipeline steel was established finally. Through the regression analysis, the model had been proved to have great matching precision.

Abstract: The samples of 9Ni steel were treated by Quenching + Tempering (QT) and Quenching + larmellarizing + Tempering (QLT). The morphology, thermal stability and mechanical stability of the reversed austenite in 9Ni steel were studied by TEM, XRD, EBSD, subzero treatment, three-point bending, uniaxial tension and uniaxial compression. It turns out that blocky reversed austenite occurs in QT-treated steel, while blocky and filmy reversed austenite occurs in QLT-treated steel, additionally, the filmy reversed austenite distributes in the lath boundaries. Subzero treatment shows that certain amount of reversed austenite in QLT-treated samples transforms after dipping into liquefied nitrogen, but it retains a higher level of more stable austenite in QLT-treated sample than that in QT-treated sample. Both tension and compression facilitate the transformation of reversed austenite into martensite, and the retained austenite mainly locates within grains.