| As one of the most typical microalloying elements which inhibits austenite recrystallization, the element Nb has been applied broadly in the production of high grade, high performance and ultra-fine grained steels. With the requirement on the properties of steel improved, the content of Nb in steels is increased. The main purpose of increasing the content of Nb is to restrain austenite recrystallization, enhance the strain accumulation and increase dislocation density so that more nucleation sites are provided for the subsequent phase transformation, thus the final microstructure is more uniform and finer, and the disadvantage effect of Nb on mixed grain can be eliminated. Recently, the technology of HTP (High Temperature Processing) has been developed rapidly; X80 pipeline steel produced by HTP will be applied in the second W-E pipeline. Therefore, it is necessary to research high Nb bearing steel systemically.In this thesis, a low carbon high Nb steel with the chemical composition of 0.05%C-0.128%Nb was designed in order to study the influence of high Nb on the microstructure, in accordance with the requirements of HTP technology, such as low carbon, high Nb and no Mo. The characteristics of dynamic and static recrystallization were investigated by means of thermo-mechanical simulator, and Tnr(temperature of non-recrystallization) was obtained. CCT curves with different deformation parameters were measured under different cooling conditions to find out the characteristics ofγ→αphase transformation in the low carbon high Nb steel. On this basis, the microstructure and property under the condition of controlled rolling and controlled cooling were assessed by the rolling experiments in laboratory. The deformation resistances were tested in different processing parameters and these mathematic simulation formulas were deduced by regression. The phase transformation kinetics of carbonitride niobium precipitation in austenite was analyzed by theoretical calculation.According to the technological features of TSCR, dynamic recrystallization of low carbon high Nb steel was studied by Thermecmastor-Z thermo-mechanical simulator, with emphasis on the dynamic recrystallization under the conditions of very coarse initial austenite grain size of 1082μm and the start rolling temperature of 1050℃. Meanwhile, the deformation activation energy of DRX for two original austenite grain sizes,1082μm and 140μm, was calculated according to the stress-strain curves, which showed that the prior-austenite grain size had little influence on the dynamic activation energy and the discrepancy was only 1kJ/mol. In order to ensure the coarse prior-austenite size and improve the reproducibility of experiments, a proper austenite grain size,400μm was adopted. According to the true strain-stress curves and microstructure, the Tnr is established as 1023℃when austenite was deformed in the condition of 2s-1 and reduction to 40%.For the double or multi-pass thermomechnically simulated rolling, the static recrystallization occuring during the inter-pass period can reduce the strain accumulation and dislocation density significantly. Thus, the static recrystallization was investigated by using double-pass compression tests with Gleeble-1500D thermo-mechanical simulator. The recrystallization fraction and the characteristics parameter, t0.5 to describe the static recrystallization kinetics has been obtained by the systematic experiments with strain rates 0.1～10s-1, reduction 10～40%, deformation temperature 850～1100℃and prior-austenite size 7.4-37.2μm. This result is necessary for the study of strain accumulation and phase transformation of low carbon high Nb steel.Deformation resistance is one of the most important parameters to determine the rolling load for the practical hot tandem rolling. Deformation resistance of high Nb steel could be different from that of low Nb steel because more dissolved and precipitated Nb exist in the high Nb steel. The deformation resistance with prior-austenite sizes of 1082pm and 140μm was measured, and the effect of hot deformation parameters such as strain rate and deformation temperature was analyzed. Then, the formulas of deformation resistance for low carbon high Nb steel with different austenite grain sizes were calculated according to the deformation resistance model with six parameters by using Matlab software. The deviation between the calculated and measured value is less than 5MPa. In addition, the deformation resistance of X65 pipeline steel (0.065%C-0.045%Nb) was also measured, aimed to compare the effect of Nb content on the deformation resistance. The results showed that the deformation resistance of low carbon high Nb steel was 25MPa higher than that of X65 for single pass deformation, indicating that the increment of deformation resistance in low carbon high Nb steel was insignificant. Theγ→αphase transformation during controlled cooling after deformation of low carbon high Nb austenite was studied by using Thermecmastor-Z simulator. The diameter change of samples during cooling was measured by the laser dilatometor attached to Thermecmaster-Z simulator. The start and finish temperatures of the transformation can be obtained from the inflection points in the recorded dilation-temperature curves. Then, the static and dynamic CCT curves of low carbon high Nb steel, which involved static transformation without deformation and transformation with single-pass or double-pass deformation, were drawn according to the inflection points. The CCT curves showed that the main microstructure is granular bainite when the cooling rate was higher than 5℃/s. Because phase transformation is closely related to prior-austenite grain size, deformation temperature, strain and strain rate, the effect of these factors on phase transformation were studied. These results showed that the increase of strain rate, decrease of soaking temperature and increase of strain could all accelerate the phase transformation, while the deformation temperature had less effect on the phase transformation. The mechanical properties of the samples produced by rolling at laboratory were measured. The minimums values of Charpy V-notch impact value of half-size sample, tensile strength (Rm), Rp0.2, and Rp0.2/Rm were 138J,642MPa,555MPa and 0.83 respectively; the total elongation was 21.5-25.5%.The precipitation kinetics of carbonitride niobium in austenite with various nucleation mechanisms, including homogeneous nucleation, nucleation at grain boundaries and nucleation at dislocations was calculated respectively. The calculation results showed that, in the case of non-deformation, the temperatures of maximum nucleation rate were 820℃,780℃and 1040℃, respectively, and the most rapid precipitation temperatures were 910℃,900℃and 1070℃, respectively, which were found to be undependant on whether the nucleation rate was constant or delined to zero rapidly. The effect of deformation stored energy within austenite on the precipitation kinetics was also calculated, and the results showed that deformation accelerated precipitation and reduced the critical nucleus size.
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