The Second Phase Precipitation Kinetics In Weathering Steel Containing Niobium

Bayan Obo iron mine is a kind of associated and coexisted deposit, which is enriched withniobium and phosphorus. The microstructure and mechanical property will be improved byproper heat treatment in weathering resistant steel with micro alloy element Nb. To studyprecipitation behaviors of niobium with different content, especial with high content inweathering resistant steel, it will be helpful for the application of niobium-bearing steeltheoretically and for the comprehensive utilization rate of Bayan Obo Ore.By optical microscope and electron microscope, the microstructure under roomtemperature and original austenite with different niobium content were observed. By usingGleeble1500D thermal simulation machine with0.1/s strain rate and30%strain capacity,isothermal iso-strain-rate axial single pass compression test was done in the range of700℃-1100℃. The influence of different deformation temperature on flow stress was studied by truestress-strain curve. With the method of stress relaxation on thermal simulator, time-stress curvecould be measured. By analyzing the inflection point of the curve, precipitation-temperature-time curve (PTT) could be drawn, which is compared with the calculated PTT curve got fromprecipitation kinetics calculation. Then NrT curve can be calculated by precipitation kineticscalculation results.The conclusion was summarized as follows:Under the condition of constant other element contents in the weathering steel, the meangrain size is decreased with the increase of Nb content. With niobium content increasing from0.042%to0.090%, the mean grain diameter is cut from5um to3.5um in microstructure underroom temperature, and from21.5um to15.3um in original austenite microstructure. In the isothermal process after deformation, the increase of Nb content don’t shorten theincubation period of strain induced precipitation. The shortest precipitation behavior is shown in900℃with0.042%Nb, and in800℃with other content niobium.In calculated PTT curve, the theoretical shortest precipitation behavior is shown in925℃with0.042%Nb, and in825℃with other content niobium. The calculation results approachesto the test results.By calculated NrT curve, the finest size of the second phase can be obtained around700℃.