| The high temperature mechanical behavior of two 0.15wt.%C steels with different concentrations of Sn is investigated. The mechanical parameters at high temperatures - reduction in area (RA%) and tensile strength ( o b) are measured by means of Gleeble-2000 thermal simulating machine. Fracture surfaces are analyzed by scanning electron microscopy and optical microscopy. The segregation of Sn is also examined by Auger electron spectroscopy. The effect of Tin and cooling rate on the high temperature mechanical properties is studied. At the same time, isothermal aging and impact testing are used to study the variations of impact energy Ak with aging time of the Sn-doped steel. The segregation mechanism of tramp element Sn during continuous cooling and isothermal aging and its influence on the hot ductility of low carbon steel are discussed and the brittleness during continuous casting of the Sn-doped low carbon steel is forecasted.At a strain rate of 10-2s-1 and a cooling rate of 10 /s, there exists a third ductility trough for both steels at 750 . The cause of the third ductility trough is the formation of film-like pre-eutectoid ferrite along austenite grain boundaries. In the third ductility region, the reduction in area of the Sn-undoped steel is larger than that of the Sn-doped steel. This means that the hot ductility of the Sn-undoped steel is better than that of the Sn-doped steel. The temperature at which the RA% is 60% is about ~850 for the Sn-undoped steel and ~910 for the Sn-doped steel. The width of the ductility region of the Sn-doped steel is obviously larger than that of the Sn-undoped steel. Addition of Sn deteriorates the hot ductility, deepens the ductility trough, and shifts the ductility region to higher temperatures.Based on the hot tensile test results, there exists a cooling rate at which a minimum reduction in area occurs and the hot ductility of the Sn-doped steel is worse than that of the Sn-undoped steel. When the cooling rate is lower or higher than 10 /s, the hot ductility is increased.The results of Auger electron spectroscopy indicate that Sn segregation occurs during continuous cooling and the degrees of segregation are different for different cooling rates. At a cooling rate of 10 /s, the maximum Sn segregation andthe worst hot ductility occur. When the cooling rate is lower or higher than 10 /s, the segregation is diminished. There is non-equilibrium grain boundary segregation of Sn during continuous cooling and the quantity is different with different cooling rate. That is to say, Sn is segregated to grain boundaries in 0.15%C steel during continuous casting.During isothermal aging, impact energy rapidly decreases and embrittlement rapidly increases with increasing aging time. When the aging time reaches 120 seconds, the minimum impact energy is obtained where the embrittlement attains the largest. As the aging time increases further, the impact energy is gradually increased and the embrittlement is gradually improved. Changing of impact energy with time responses of the kinetics of non-equilibrium segregation, so the time at which the impact energy is minimum is measured as the critical time. As measure in this experiment the critical time of Sn non-equilibrium segregation is 120 seconds at 750 .Based on these experiments, it may be claimed that there is segregation of Sn to grain boundaries in 0.15%C steel during cooling from a high solution treatment temperature. This segregation reduces grain boundary cohesion, which results in low hot ductility and intergranular fracture. This segregation behavior of Sn accords with the mechanism of non-equilibrium segregation. It can be obtained through the theoretical calculations that the critical time of Sn non-equilibrium segregation is 124 seconds at 750 and the critical cooling rate is 8 /s when cooling from 1320 .It is suggested that controlling the cooling rate during continuous casting to avoid the critical cooling rate (8 /s) and avoiding high stress near the trough temperature (750 ) may control the form...
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