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In order to eliminating the banded structure in SAE8620H gear steel,an isothermal annealing process and its mechanism has been studied in this paper. Results showed that isothermal annealing process can effectively eliminate banded structure in SAE8620H gear steel.With isothermal temperature 640°C-a certain low temperature indeed high undercooling,and for high nucleation at this temperature resulting in the decrease of nucleation rate difference in all areas,banded structure level decreased to 1 level.When isothermal temperature decreased to 610°C and 590°C,although the undercooling is so high that various regions began to nucleate at the same time,the transition temperature is so low that alloy elements in the regions previously occupied by pearlite bands diffuse faster than other regions,thus proeutectoid ferrite grains in these regions grow faster,regenerating the banded structure .

By adding Mn into SAE8620 gear steel, the effect of Mn on the microstructure and hardness of steel treated by quenching and partitioning process is investigated in this research. Microstructure is observed. Hardness is measured, after that it is converted into tensile strength. And the content of austenite is determined by X-ray diffraction analysis. It is found that higher content of Mn increases the content of martensite and reduces that of blocky ferrite and also leads to higher hardness of steel. In addition, XRD pattern shows obvious existence and enhancement of retained austenite.

Material cutting properties can be judged by turning temperature and chip shape. A new method that can measure turning temperature more accurately was discussed in this paper. Compared with the traditional method, results show that turning temperature can be accurately measured through a new method in turning process. Turning temperature become stable and will not jump after 20s later. In addition, the higher the hardness, the higher the energy is needed, leading to the higher turning temperature. The average turning temperature of the quenched specimen, normalized specimen and annealed specimen measured through the new method is 100°C,70°Cand 60°C, respectively. And the chip morphologies of quenched specimen, normalized specimen and annealed specimen are irregular band chips, spiral debris chips and C-shaped chips, respectively.

Non-isothermal resistance heating in the hot stamping of quenchable steel sheets was developed to produce ultra-high strength steel formed parts with tailored properties. The heating temperature of parts is related with width of samples heated by resistance heating. With the same input energy, the strength in the narrow portions is high owing to the high energy density and that in the wide portions is low owing to the low energy density. Hat-shaped products having a tensile strength arrange from 600 MPA to 1800 MPa were formed. The tempering treatment on the directly hot-stamped boron steel resulted in better mechanical properties and higher formability index. The SEM figures indicates that the nano-carbide formation during the tempering treatment were suggested as the evident reasons for the occurrence of the mentioned robust properties. Finally the combination of temperature 250 ℃ and holding time 45 min can achieve the best comprehensive mechanical properties.

In this study, a improved mathematical model was developed for Nb-Ti microalloyed steel during hot rolling simulation. Using the compression test, the dynamic and static recrystallization characteristics of Nb-Ti microalloyed steel were studied. Though multi-pass hot rolling simulation, it is found that the recrystallization during hot rolling can play an important role, it can make the mean flow stress lower and refine the grains. And respective comparison between calculated and measured data of microstructure showed some of the validation of the built model. Meanwhile, the evolution characteristic of average austenite grain size during hot rolling can be achieved by theoretical model and experiment.

Technology and defect in the low-pressure semi-solid die casting of aluminum wheel hub under different process has been studied. The simulation software ADSTEFAN 2012 is used to analyze the solidification in different die temperature. Stereomicroscope, digital camera and optical microscope (OM) are used to study defect in the casting. The result shows that the low slurry temperature (606°C) will result in higher viscosity, and the low preheated die temperature will increase the cooling rate so that shrinkage is formed in the isolate solidification area. In the technology of casting wheels, the optimum temperature of the slurry is 614°C, preheated temperature of die is 400°C.

Austenitic continuous heating transformation dynamics, microstructural evolution and quenching properties of 22MnB5 steel during resistance heating were investigated. Steel sheets with thickness of 1.6 mm were heated to various temperatures (800~1200 °C) either by resistance heating at two heating rates of 100 and 300 °C/s or furnace heating. The increment of superheat degree was parabolic relationship with increasing heating rate. Samples heated above Ac3 of respective heating rates exhibited a full martensite microstructure and a hardness of above 490 HV. Austenitic coarsening rate of resistance heating samples was much lower than those by furnace heating. Austenite grain size by resistance heating was smaller than 38 μm while these by furnace heating reached 74 μm at 1200 °C. Samples heated by resistance heating at 300 °C/s had a good performance of 1800 MPa tensile strength and 5.6 % elongation, and product of strength and elongation reaches 10080 MPa%. The good combination of tensile strength and elongation by resistance heating might result from the finer austenite grain size and smaller martensite plate.