Oriented Silicon Steel, Copper Sulfide Precipitation Form The Basis Of Its Solid Solution

The grain oriented silicon steel which has excellent magnetic properties along the rolling direction, is essential material for manufacturing the core of transformer. It is the only products manufacted by using the phenomenon of secondary recrystallization in iron and steel industry. The traditional high temperature slab reheating technology for grain oriented silicon steel no longer meets these requirements such as environmental protection, energy conservation and reducing costs in the modern steel industry. Copper sulfide (CuxS) is a kind of grain inhibitor which can decrease the reheating temperature of slab. There is still controversial currently about the precipitation form of copper sulfide grain inhibitors in grain oriented silicon steel. Therefore, it is very important to investigate the specific form of CuxS precipitate playing an important role in grain oriented silicon steel. In this paper, the density of states, band structure, formation energy and binding energy of copper sulfides such as.Cu1.8S, CU2S and CuS which may exist in grain oriented silicon steel with copper are calculated and analysed using the first principles calculation module of CASTEP and DMOL3 of Materials Studio based on the density functional theory of quantum mechanics, and the most possible form of the precipitation of copper sulfides is investigated. Furthermore, the precipitated second phase particles in tested grain oriented silicon steel with copper are investigated by TEM. Meanwhile, based on the studies of first principles and experiment, the formulas of solubility of copper sulfides in the iron matrix are derivated by thermodynamic method.The calculation results show that the average binding energy of Cu1.8S,Cu2S and CuS are-10.12416 eV,-10.16943 eV and-6.91536eV, respectively.It can be seen that the phase stability of three kinds of copper sulfide is decrease by the order of Cu2S,Cu1.8S,CuS. The formation energy of Cu1.8S,Cu2S and CuS are negative, and their value are-97.7594 eV,-1.2055 eV,-0.9699 eV, respectively.The results show that they are thermodynamically stable phase,and the formation energy of Cu1.8S is the most negative value which indicates that Cu1.8S has the highest forming ability.The calculation results of density of states show that the density of states of Cu1.8S,Cu2S and CuS at fermi level are all higer than zero,1.65 electrons/eV for Cu2S,2.58 electrons/eV for Cu1.8S and 2.52 electrons/eV for CuS, respectively. The Cu2S with the most stable structure is due to that it has the minimum value of density of states at fermi level.The results of TEM eperiments for the second phase in the tested grain oriented silicon steel which contains relatively high content about 0.48%Cu show that the fine particle precipitations along the grain boundaries is Cu1.8S with cubic crystal structure.The balance solubilities of CuS and Cu2S in iron matrix have be deduced by using thermodynamic method. Futhermore, it can make conclusions by comparison of the figures of the balance solubility in the iron matrix that the balance solubilities of Cu2S in ferrite and austenite are less than that of CuS, and the precipitation amount of Cu2S in grain oriented silicon steel is more than that of CuS in the case of Cu-rich. However, it is difficult for precipitating CuS in the grain oriented silicon steel because of the results of competition.It can be inferred by the results of the first principle study and comparison of solubility product that it is easier that the relatively stable structure of Cu1.8S or Cu2S as grain inhibitor precipitate in grain oriented silicon steel. However, the copper sulfides in grain oriented silicon steel often precipitate in the hot rolling process or during the normalizing annealing(about 1100℃). Therefore, the really effective grain inhibitor of copper sulfides precipitated at high temperature is the Cu1.8S with cubic crystal structure.