| Continuous casting and rolling are an important revolution in the twentieth century in the steel industry, it can improve the rolling yield of materials and reduce the cost of production. As the scale of the power station is increasing these years, the station reactor core and pressure vessel diameter and thickness are increasing. In order to improve the hardenability of the steel, a certain amount of alloy should be added in the steel during the process of continuous casting. Research shows that the carbon content increases in aluminized steel could reduce the precipitation of carbonitride aluminum at the grain boundaries and refine the austenite grain size, result the increased hot ductility. But the researches about the effects of rare earth elements on steel hot ductility are less studied. Besides the alloying element, some trace elements as phosphorus and tin which are difficult to remove during the process of continuous casting. As there are two different opinions on the effects of phosphorus on the steel hot ductility, it is necessary to study the effects of phosphorus on the hot ductility of steel. In addition, studies about the combined effects of those trace elements on the hot ductility of steel are little. The transverse cracking is consistent with the hot ductility trough, so the thermal simulation tensile test could be explored to simulate the continuous casting process, which can explain the reason of the occurring of the transverse crack, then find out the solutions.In this thesis, two types of low alloy pressure vessel steel, i.e., Cr-Mo steel and C-Mn steel would be used to exceed the thermal simulation tensile test, the effects of the trace elements of phosphorus, tin and cerium on the hot ductility of steels should be studied. Besides, the combined effects of the phosphorus with tin and the cerium with tin on the hot ductility of the low-alloy pressure vessel steel should be studied as well. The main research contents and results are as follows.A Gleeble-1500 D thermomechanical simulator was used to simulate the continuous casting process for the undoped, P-doped, Sn-doped, P+Sn-doped, Ce-doped and Ce+Sn-doped 1Cr-0.5Mo steel and undoped, Sn-doped and Ce+Sn-doped C-Mn steel. The steel samples undoped and doped with different elements are heated at 1300 for 3 min and the cooled at a rate of 5 s-1 down to different test temperatures, followed by tensile deformation until fracture. Then the scanning electron microscopy(SEM), the optical microscopy(OM), the electron backscatter diffraction(EBSD), the field emission gun scanning transmission electron microscopy(FEGSTEM) were used to analyze the microstructures of the fracture zone, the nano-indentation was employed to measure the hardness of ferrite along the grain boundary, and the FEGSTEM was used to measure the P, Sn and Ce grain boundary concentrations of different low-alloy pressure vessel steel deformation zone. The results show that the hot ductility of the steel, evaluated by the reduction in area, can be substantially enhanced by the addition of phosphorus. Nanohardness measurements and microscopy indicate that, in the austenite-f rrite regions, the hot ductility improvement is mainly caused by the phosphorus-ind ced solid solution hardening of ferrite layers formed along austenite grain boundaries, and over 900, the phosphorus-promoted dynamic recrystallization of austenite so as to increase the hot ductility. And the hot ductility of the steel can be substantially deteriorated by tin, but considerably enhanced by phosphorus and tin. Therefore, the detrimental effect of tin on the steel hot ductility is completely suppressed by the beneficial effect of phosphorus. Grain boundary chemistry measurements indicate that the hot ductility deterioration by Sn may mainly arise from its grain boundary segregation. Nanohardness measurements and microscopy demonstrate that, in the austenite-ferrite region, the hot ductility enhancement by phosphorus and tin is mainly caused by the phosphorus-induced solid solution hardening of ferrite layers formed along austenite grain boundaries, and in the austenite region, by the phosphorus-promoted dynamic recrystallization of austenite.The hot ductility of a 1Cr-0.5Mo low alloy steel is increased by the addition of rare earth element cerium. In the austenite-ferrite dual-phase region, cerium may delay the formation of proeutectoid ferrite layers along austenite grain boundaries, thereby increasing the hot ductility of the steel. In the single austenite region, grain boundary segregation of cerium may increase the grain boundary cohesion, toughening the steel and thus raising the resistance to grain boundary sliding as well as promoting dynamic recrystallization. Consequently, the hot ductility of the steel is enhanced.A certain amount of cerium added in tin-doped Cr-Mo steel could increase its hot ductility. The segregation of cerium could resist the grain boundary sliding, the negative effects of impurity elements tin on the hot ductility of steels could be inhibited with cerium. And a certain amount of cerium added in tin-doped C-Mn steel could also increase its hot ductility. In the austenite ferrite two-phase region, impurity elements tin can increase the Ae3 temperature in the formation of pro-eutectoid ferrite, leading the hot ductility trough of the tin-doped steel appeared in advance, and cerium can induce the dynamic process of ferrite to postpone the ferrite by the influence of deformation. At the same time, cerium atoms segregation at the grain boundaries, thereby inhibiting grain boundary sliding, promote the dynamic recrystallization process. |
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