CaF2 is used in large quantity during refining of steel, which is harmful to environment to be found. There is more and more study on use of B2O3 in metallurgical progress. This paper studied the effects of MgO on melting performance of CaO-SiO2-Al2O3 ternary system and B2O3 on that of CaO-SiO2-Al2O3-MgO quaternary system by calculation of phase diagram. The result shows that MgO can expand the range of liquid phase of CaO-SiO2-Al2O3 ternary system while the effect on melting performance is little. B2O3 can also reduce the melting point of CaO-SiO2-Al2O3-MgO quaternary system in high CaO zone and in high Al2O3 zone. So the proper composition of MgO in refining slag are 5~8% and B2O3 below 8%.
To improve the performance of NM400, V, Ti and Ce were added in NM400. The effect of alloy on grain size, precipitation, hardness and toughness was studied. The result shows that V and Ti can improve the performance by grain refiner and dispersion-strengthening. By further adding Ce, the size of grain and precipitations of V and Ti is significantly reduced, the solid solution quantity of V and Ti and the amount of precipitation are increased, and the precipitation shape transforms from cube to sphere. Therefore the hardness and toughness of steel are greatly improved. In this study, 0.2% V and 0.2% Ti were added into NM400. As a result, the precipitation of V and Ti, in form of carbide and nitride, was the smallest and dispersed, and the grain size was the smallest, which leaded to the best performance. The conclusion is that, to ameliorate the performance of NM400, the optimal additive amount of V and Ti is 0.2% no matter whether adding Ce or not.
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