As the raw materials in the post process of rolling and heat treatment, ingots have great effects on the properties of the final products. Inclusions and solidification structures are the most important aspects of the quality of ingots. Niobium and titanium are usually used to react with carbon and nitrogen to improve the properties of ferritic stainless steels. In this research, combined with thermodynamic calculation, effects of niobium and titanium on the inclusions and solidification structures in three kinds of high pure ferritic stainless steels with different titanium additions were investigated by optical microscope（OM）, scanning electron microscope（SEM）, transmission electron microscope（TEM）, and energy disperse spectrometer（EDS）. Results show that Al2O3 and a few（Nb,Ti）N particles form when titanium addition is 0.01 %.Furthermore, inclusions are mainly Ti N and Al2O3–Ti Ox–Ti N duplex inclusions when titanium addition is more than0.10 %. Those two types of inclusions are in well distribution, and can afford nuclei to the solidification process.Therefore, the ratio of equiaxed zone increases with the increase of titanium addition. The ratio increases from42.1 % to 64.0 % with the titanium addition increasing from 0.01 % to 0.10 %, and it increases to 85.7 % when the titanium addition reaches 0.34 %.
As stabilizing elements added into ultra-pure ferritic stainless steels, niobium and titanium react with car- bon and nitrogen to form carbonitrides and have great effects on the ratio of equiaxed zone and the grain size of solidi- fication structure of ingots, which remarkably affect the quality of cold-rolled sheets. Combined with thermodynamic calculation, style and precipitation progress of inclusions in ultra-pure ferritic stainless steels were investigated by optical microscopy, scanning electron microscopy, transmission electron microscopy and energy dispersive spectros- copy. The results indicate that the inclusions are mainly Ti-Al-N- O system inclusions in ultra-pure ferritic stainless steels. Al2Oa starts to precipitate firstly and then TiOx and TiN precipitates sequently. The inclusions are mainly single TiN particles and complex inclusions with Al2O3-Ti2O3 as cores and covered with TiN under the condition of 0.31% titanium addition and mainly Al2O3 under the condition of 0.01% titanium addition. A few （Nb,Ti）N parti- cles precipitate because of no enough titanium to react with nitrogen when titanium addition is 0.01 %. In addition, fine Nb（C, N） particles with size of less than 500 nm precipitate at relatively low temperature.
The single hot thermocouple technique （SHTT） and high temperature equilibrium technique were combined to investigate the phase diagram of the CaO-SiO2-5%MgO-20%AlzO3-TiO2 system. The 1300 ℃ to 1500 ℃ liquidus lines are calculated according to the thermodynamic equations based on the pseudo-melting temperatures measured by the single hot thermocouple technique. The phase equilibria relationships are experimentally determined at 1400 ℃ using the high temperature equilibria technique followed by X-ray fluorescence （XRF）, X-ray diffraction（XRD）, scanning electron microscopy （SEM） and energy dispersive X-ray spectroscopy （EDX） analysis. The liquid phase（L）, melilite solid solution phase （（C2MSz,C2AS）ss）, diopside phase（CMS2） and perovskite phase （CaO·TiO2） are found. Coupled with the liquidus lines and equilibria results, the phase diagram is constructed for the specified region of the CaO-SiO2-5%MgO-20%Al2O3-TiO2 system.
A coupled thermodynamic model of inclusions precipitation both in liquid and solid phase and microseg- regation of solute elements during solidification of heat-resistant steel containing cerium was established. Then the model was validated by the SEM analysis of the industrial products. The type and amount of inclusions in solidifica- tion structure of 253MA heat-resistant steel were predicted by the model, and the valuable results for the inclusions controlling in 253MA steel were obtained. When the cerium addition increases, the types of inclusions transform from SiO2 and MnS to Ce2 O3 and Ce2O2 S in 253MA steel and the precipitation temperature of SiO2 and MnS decrea- ses. The inclusions CeS and CeN convert to Ce2 O3 and Ce2 O2 S as the oxygen content increases and Ce2 O3 and CeN convert to Ce2 O2 S, Ce3 S4, and MnS as the sulfur content increases. The formation temperature of SiO2 increases when the oxygen content increases and the MnS precipitation temperature increases when the sulfur content increa ses. There is only a small quantity of inclusions containing cerium in 253MA steel with high cleanliness, i. e. , low oxygen and sulfur contents. By contrast, a mass of SiO2 , MnS and Ce2 O2 S are formed in steel when the oxygen and sulfur contents are high enough. The condition that MnS precipitates in 253MA steel is 1.2wEo[O] ＋W[s]〉0. 01% and SiO2 precipitates when 2w[O] ＋wrs[S]〉0. 017% （W[S]0. 005%） and w[O]〉0. 006% （w[S]〉0. 005%）.
