Study on the deformation mechanism of inclusions containing sodium in cord steel

Abstract

Large-sized hard inclusions led to the problem of wire breaking during the cold drawing process. So it is very important to plasticise inclusions in cord steel. In this article, the plastic modification mechanism of Na₂CO₃ on inclusions in cord steel was studied by adding different contents (0%, 0.2%, 0.4%, 0.6%) Na₂CO₃ in a MoSi₂ resistance furnace. The ingots were forged to investigate the deformability of inclusions. The size of inclusions during the cold drawing was the same as that after forging because the cold drawing was carried out at room temperature. Therefore, the size and deformability of inclusions after forging can be used to measure the wire breaking rate of cord steel wire during the cold drawing process. Through the analysis of ingot samples and forging samples by SEM-EDS, it was found that the deformability of inclusions was improved and the inclusions were in the form of long strips or broken after forging. In addition, not only the low melting point phase diagrams of SiO₂–Al₂O₃–MnO–(15%/10%)CaO–(0%–9%)Na₂O but also the activities of SiO₂, Al₂O₃ and MnO were calculated by FactSage8.2. It was found that Na increased the low melting point region. When the contents of [Si], [Mn] and [Al] in molten steel were determined, the values of $a_{{\mathrm{Al}}_2{\mathrm{O}}_3}^2/a_{{\mathrm{SiO}}_2}^3$ and $a_{{\mathrm{SiO}}_2}/a_{\mathrm{MnO}}^2$ remained constant according to metal-inclusion thermodynamic equilibriums 3SiO_2(inc) + 4[Al] = 2Al₂O_3(inc) + 3[Si] and [Si] + 2MnO_(inc) = 2[Mn] + SiO_2(inc). Under the condition of constant $a_{{\mathrm{Al}}_2{\mathrm{O}}_3}^2/a_{{\mathrm{SiO}}_2}^3$ and $a_{{\mathrm{SiO}}_2}/a_{\mathrm{MnO}}^2$ , the activity coefficients of SiO₂, Al₂O₃ and MnO were changed by adding Na₂O, which led to the change of inclusions component. Therefore, it was obtained the effect of Na₂O content on inclusion composition and melting point using FactSage8.2 thermodynamic calculation and low melting point region calculation of phase diagrams. The result was that the inclusion deformability was well improved during thermal deformation under the condition of about 0.4% Na₂CO₃. Thus the wire breaking rate was reduced. It is expected that this work can provide theoretical guidance for the industrial production of feeding alkali wire in the later stage of refining.