Control Of Non-metallic Inclusions In Sulfur-containing Steel

It is required to improve the machinability of steel along with the rapid developing of mechanical processing industry and high speed, precision, automated level of machining process. Sulfur-containing steels are now widely applied in machining industries due to the good cutting property. The main factories producing sulfur-containing steel are special-steel enterprises in our country. The technology is not perfect and only a few factories can product stably. At present, the poor castability is bottleneck in sulfur-containing steel production. Especially, the clogging is a common phenomenon in the continuous casting of high sulfur steels, which is caused by inclusions. So, It’s significant to investigate the inclusion evolution in different alloying process for determing a reasonable alloying process and solving the problem of nozzles clogging.In this research, the thermodynamic calculations was carried out about the evolution of some main types inclusions during making sulfur-containing steel. On the other hand, the laboratory experiments have been carried out to study the effects of different alloying process (Al, Ca, Mg, S) on inclusions by using the MoSi2electric resistance furnace. The effect of alloying process on morphology, composition and size of non-metallic inclusions in steel was investigated by metallographic microscope and SEM. And the influences of alloying process on the sulfur yield is studied simultaneously. Based on the thermodynamic calculation and laboratory experiments, industrial experiment was carried out and good results were achieved.The conclusions are as following:(1) Analysis of the clogged nozzle and inclusions in tundish shows that Al2O3and CaS inclusions cause nozzle clogging.(2) The thermodynamic calculations shows following results:the dissolved oxygen concentration can reach8×10-6when Al content is about0.02%; CaS forms easily with high sulfur content in steel, and high temperature can inhibits the formation of solid CaS. High calcium content is needed to form liquid calcium aluminate when modify Al2O3. when w[Al] is0.02%, the value of w[S] should be controlled below0.016%in order to produce liquid CaO-Al2O3inclusions avoiding the formation of solid CaS; when dissolved aluminum content is0.02%, MgOAl203spinel inclusions can form easily even if a little of Mg exists in steel. Therefore, it’s feasible to modify Al2O3inclusions by magnesium treatment in sulfur-contenting steel.(3) The experiments show following results:the sulfur yield is higher when sulfur is feeded after modifying inclusions, and the yield is highest taking Magnesium treatment to modify inclusions. The most inclusions in molten steel are Al2O3-CaO and Al2O3-CaO-CaS for calcium treatment, and Al2O3-MgO and pure MgO for Mg treatment. Pure CaS and Al2O3inclusions don’t exist in the molten. Especially during the process taking Mg treantment, the inclusions size are very fine and the edges are smooth, which shows better effects of modification.(4) Industrial experiment shows that the nozzle clogging can be inhibited by increasing the Ca-wire amount, extending the gap between the calcium treatment and sulfur-feeding, and some other method. Continuous casting heats increase from4to7after adopting the measures above.