Di Tang

Influence of Cold Rolling Reduction on Microstructure and Mechanical Properties of Twip Steel

The influence of cold rolling reduction on microstructure and mechanical properties of the TWIP (ttwinning induced plasticity) steel was investigated. The results indicated that the steel had better comprehensive mechanical properties when cold rolling reduction was about 65.0% and the annealing temperature was 1000°C. The tensile strength of the steel is about 640MPa and the yield strength is higher than 255MPa, while the elongation is above 82%. The microstructure is composed of austenitic matrix and annealing twins at room temperature, at the same time, a significant amount of annealing twins and stacking faults are observed by transmission electron microscopy (TEM). Mechanical twins play a dominant role during deformation, and result in excellent mechanical properties.

Static Recrystallization Kinetics Model of X70 Pipeline Steel

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.

Thermal Stability of Non-Equilibrius Microstructure in Microalloyed Steel During Reheating

Cooled in water after isothermal relaxation of deformed austenite for different times, an Nb-bearing microalloyed steel always exhibits synthetic microstructures, in which bainitic ferrite dominates. Dislocation configurations and distributions of strain induced precipitates inside bainitic ferrite of samples relaxed for different times were distinct. When compared with the austenite model steel, which maintained fcc structure even at room temperature, the strain induced precipitates were not found in the sample without relaxation whereas these were distributed outside dislocations in sample relaxed for 1000s. Most of the strain induced precipitates distribute along dislocations and pin dislocations in sample relaxed for appropriate time. After bainitic transformation, the dislocations formed in deformed austenite remain to be pinned by the precipitates. When these samples were reheated to and held at 650 or 700 °C, the non-equilibrious microstructures tended to evolve into equilibrious ones. The sample relaxed for 60s displayed the highest thermo-stability, whereas microstructure evolution was the quickest in the sample relaxed for 1000s even though it was the softest prior to reheating. Dislocations inside laths got rid of pinning of precipitates, and their polygonization became the precursor to the evolution of microstructures during reheated and held, followed by gradual disappearance of lath boundaries caused by dislocation climbing. Finally, recrystallization occurred and polygonal ferrite appeared. By hardness measurement, it was found that softening is not a single process occurring during reheated, in which hardness fluctuates with time. There were two peaks in the hardness-time curve of each sample having undergone relaxation, while single peak occured in the curve of the sample not being relaxed. These results indicated that the thermo-stability of microstructures was determined by their history of formation to a considerable degree.

Study on High-Cycle Fatigue Property of Q500q Bridge Steel

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.

Stability of reversed austenite in 9Ni steel

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.

Mathematical Model of Hot Deformation Resistance of X120 Pipeline Steel

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.

Effects of Continuous Annealing Process on Microstructures and Properties of C-Mn-Si Bearing Cold-Rolled TRIP Steel

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%.

Mechanical Properties and CO2 Corrosion Behavior of Q125 Grade Oil Tube Steel Used in Deep Oil Well

A type of Q125 grade high-strength low carbide oil tube steel was designed and researched. The strengthening mechanism of the steel was analyzed by SEM and TEM, and the CO2 corrosion behavior of the steel was simulated by high-temperature and high-pressure autoclave. The result show: the size of original austenite grain, packet and block are about 10-15μm, 2μm and 200nm, respectively, so the strength of the steel matched toughness well. The precipitate of (Nb,Ti)(C,N) whose size is about 60nm can restrain coarsening of the original austenite grain through pitting austenite grain boundary; The precipitate of (Nb,Ti)C and nanometer sized precipitate of NbC can be an effective barrier for dislocation motion, so that has great contributions to improve the strength of the steel. The average and pitting corrosion rate increased first and then dropped down with the increasing of temperature. There is a maximum of average and pitting corrosion rate at 90°C; Simultaneously, when the temperature is 90°C, there is a biggest difference between average corrosion rate and pitting corrosion rate.

Effect of Heat Treatment Process on Microstructure and Properties in Low Carbon Si-Mn Q&P Steel

The mechanical properties of Low Carbon Si-Mn Q&P steel are strongly affected by the conditions of heat treatment. Microstructures and mechanical properties of Low Carbon Si-Mn Q&P steel at different partitioning temperature and holding time was investigated. The microstructure was analysed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It is shown that the microstructure of Q&P steel is carbon-depleted lath martensite and carbon enriched retained austenite. The retained austenite appear film-type between the laths. Higher partitioning temperature and longer partitioning time can obtain more retained austenite. It is shown that with increasing partitioning time ultimate tensile strength decreases, while elongation increases obviously. Carbon-enriched metastable retained austenite is considered beneficial because the TRIP phenomenon during deformation can contribute to formability and energy absorption.

Effect of Different Coiling Temperature on Mechanical Properties in Hot Rolled TRIP Steel

By means of optical microscopy(OM), scanning electron microscopy(SEM),X-ray diffraction(XRD),And tensile test, Mechanical Properties of hot rolled transformation -induced plasticity (TRIP) steels which were prepared through three different coiling temperature was investigated. Result reveals that the formability index of the experimental steel descends when the coiling temperature becomes low. Different coiling temperature has greater impact on retained austenite. Amount and carbon content of retained austenite in the experimental steel get less with lower coiling temperature.