Effect of Zr Treatment on Inclusion and Solidification Microstructure in Ti Deoxidized Low Carbon Microalloyed Steel

Inclusion-induced acicular ferrite (AF) is a preferred microstructure that could refine grains and improve strength and toughness. As a prerequisite, the effective inclusions generated by the optimal range of Zr mass fraction are essential for refining inclusion size and obtaining AF in Ti deoxidized low-carbon microalloyed steel. Scanning electron microscopy, electron probe micro-analysis, and electron backscattered diffraction were used to systematically investigate the effect of Zr treatment on the inclusion and solidification microstructure. The mechanisms of AF nucleation induced by different types of effective inclusions were also revealed. The results show that when the Zr mass fraction is 0.0020%, the inclusion size is refined, the effective inclusions are TiN and Ti2ZrO6, the AF volume fraction and HAGB proportion reach the maximum value, and the effective grain size reaches the minimum value. The low lattice mismatch is the main mechanism of AF nucleation induced by TiO, TiN, and TiN containing composite inclusions. AF nucleation induced by Ti2O3, Ti2ZrO6, and other oxides is mainly the MDZ mechanism. Considering the inclusion characteristics and solidification microstructure, the optimal control range of Zr mass fraction in Ti deoxidized low-carbon microalloyed steel is 0.0020-0.0046%.