The Evolution of Particles during Solidification of Nb-Ti micro-alloyed Steels and Mn/Si deoxidized

Abstract

The evolution of particles during solidification of steels was investigated by experiments and microsegregation calculations employing simple microsegregation models with a help of computing software such as ThermoClac and FactSage for the better control and utilization of inclusions and precipitates. In order to elucidate the formation of large Nb-Ti carbonitride particles which cause defects in final steel products such as linepipes or shipbuilding plates, the formation of Nb-Ti carbonitride particles during solidification was investigated employing unidirectional solidification technique with a constant cooling rate (8？aC/min) and the effects of microsegregation was estimated using simple microsegregation models.During unidirectional solidification, most (Nb,Ti)(C,N) particles were formed in the interdendritic region due to microsegregation and their size and morphologies were affected by the steel composition. With increasing alloying elements such as Ti, Nb, C, and N, the size of carbonitride particles increased. Type II Cubic Ti-rich (Nb,Ti)(C,N) particles were heterogeneously nucleated on the surface of oxide inclusions at earlier stage of solidification than Nb-rich (Nb,Ti)(C,N) particles. Type III Plate- and type IV fishbone-like Nb-rich (Nb,Ti)(C,N) particles were formed at the final stage of solidification. Unintended formation of type I dendritic Nb-rich (Nb,Ti)(C,N) particles occurred due to solidification of the remaining liquid steel during quenching stage.Under the solidification condition of the present study, modified Scheil model considering complete diffusion of interstitial elements such as C and N was well applicable to simulate microsegregation and formation of (Nb,Ti)(C,N) in the interdendritic liquid. Considering macrosegregation in the slab center, with increasing macrosegregation degree, size of (Nb,Ti)(C,N) increased. Especially, size of Nb-rich (Nb,Ti)(C,N) particle sharply increased when the macrosegregation degree exceeded a certain value. In order to utilize oxysulfide inclusions more efficiently in the free-cutting steels, phase equilibria of the MnO-SiO2-Al2O3-MnS quaternary system in the temperature range of 1185 to 1500？XC was measured and behavior of oxysulfide inclusions which consist of the quaternary system was investigated in the unidirectionally solidified steel. In the MnO-MnS system, the eutectic temperature and composition of the MnO-MnS system were also determined to be 1256∂3？XC and 64∂1mass% MnS. In the MnO-SiO2-MnS system, ternary compound ¨A〃(67.3mass% MnO- 23.4mass% SiO2- 9.3mass% MnS) which melted incongruently at a temperature between 1280 and 1300？XC and ternary compound ¨B〃(74.4mass% MnO- 22.0mass% SiO2-3.6mass% MnS) which melted incongruently at a temperature between 1300？XC and 1315？XC and decomposed into MnO, Mn2SiO4 and the ternary compound ¨A〃 were observed. Based on the experimental results, polythermal projection of the MnO-SiO2-MnS system below 1500？XC was estimated. In the MnO-SiO2-Al2O3-MnS system, the MnS solubility is strongly affected by the Al2O3 content and also the MnO/SiO2 ratio. It increased as increasing MnO/SiO2 ratio at a constant Al2O3 content and decreased as increasing the Al2O3 content at a constant MnO/SiO2 ratio. In addition, depending on the Al2O3 content and the MnO/SiO2 ration, the liquid phase is possibly saturated with not only MnS but also other solid oxide phases. In the aspect of inclusions utilization for free-cutting steel, it might have an advantage to decrease Al2O3 content and increase MnO/SiO2 ratio.Thermodynamic calculation of the MnO-SiO2-Al2O3-MnS system using FactSage with newly developed database by Kang and Pelton shows a good agreement with the results of present study. Therefore, it was confidently used for the calculation of phase equilibria in the oxysulfide inclusion during solidification steels.The behavior of oxysulfide inclusions was investigated employing unidirectional solidification technique and the effect of microsegregation was estimated using Scheil model with a help of thermodynamic calculation software (FactSage).The oxysulfide inclusions which were formed in the liquid before solidification had high content of Al and they were dispersed uniformly in the intradendritic region after solidification. With increasing solid fraction, Al2O3/(SiO2+Al2O3) ratio in the inclusions gradually decreased. According to the Scheil calculation, soluble Al in molten steel was almost consumed when the solid fraction exceeded about 0.5 thereby newly precipitated inclusions during solidification or after solidification had low Al content. It shows a good agreement with the experimental results. Al2O3/(SiO2+Al2O3) ratio in the inclusions which were located on the interdendritic region was much lower than the primary inclusions. In addition, oxysulfide inclusions which were precipitated after solidification had also low value of Al2O3/(SiO2+Al2O3) ratio.